The slasher as described in the Garden City General Encounters is for a specific purpose. It's the type of creature that the characters run into once, defeat, then move on. Remember, the Slasher on that blogpost only has 2 HD. They don't realize he's going to come back, start hunting their friends and family, become a nightmare, and try to kill them at every turn. That's what a Slasher does.
Slashers are people that turn themselves over to a murderous persona. They become empowered by it, and their longevity is increased. More then one amoral wannabe has taken up the knife in an attempt at immortality. But there is more to them then this. You see, while Slashers are undoubtedly a psychic phenomenon of the queer city of Garden, an actual Slasher who is psychic is unheard of. It could be that being a Slasher requires a mind that is incapable of becoming a psychic, or perhaps becoming a Slasher IS a psychic gift, just a despicable one. Even the most revolutionary psychic scholars would be unlikely to classify it as a legitimate psychic power.
However all Slashers begin as normal people, and normal people have morals. Slashers don't kill just anyone, they always begin by killing only the right type of people. They only kill gang members, they kill for revenge, they kill bums or prostitutes, or they kill people of an alien species they don't like very much. But the more they kill, the farther their standards degrade. Eventually Slashers become so numb they'll kill almost anybody once their back is turned, retreat from any kind of normal life, and become totally outcasted. Taking the mask off at this point feels less like the real them then leaving it on. Eventually the Slasher ceases to be a person at all- their house left abandoned and they simply stalk the night. They still have territory and lair they stalk, but they no longer can communicate or care about anything but killing.
As Slashers kill, they become stronger. This general level of strength isn't always constant, as Slashers can become stronger against certain people by hunting them down and increasing the fear and suffering they cause that person. Slashers with 1 HD are barely starting out, and have limited abilities. Slashers with 2-4 HD are legitimate slashers now, starting to lose themselves or having already lost themselves to their persona. At this point, even after you defeat a Slasher, you may not be able to remove the mask from their face. It may be stitched or melded with their face directly, overtaking their entire being. Slashers at this point also start displaying their supernatural abilities such as super stealth, disappearing when nobody is looking, and incredible endurance; able to come back even after confirmed to be dead. Slashers with 5-8 HD or more are truly terrors of the night, and have an almost mythic reputation among the city's populace.
The Slasher's ability to come back from the dead is their most fel power and what helps along their immortality. Lesser Slashers of the 1-2 HD variety are simply tougher then most people and are capable of coming back after a few stab wounds or shotgun blasts. Unusual, but not impossible. Slashers of the higher tiers of power break reality much more. These Slashers may be able to recover incredible amounts of damage, even decapitation, blowing them apart, or watching them burn to ash may not stop them. At this stage they can return as a sort of semi-spirit, weakened but still capable of killing and becoming stronger. Even stronger Slashers may return fully corporeal immediately, or have a body that regenerates itself as soon as it is damaged and put themselves back together.
It is still possible to end a Slasher; and a few madmen in Garden do it regularly to make money or just to make the city a better place to live. Defeating a Slasher requires the hunters to have no fear, and to keep their family and friends safe as the Slasher will seek revenge. While enough damage can kill lesser Slashers, more powerful ones will require many repeated gruesome deaths to put down or one of these advanced methods. Since one won't always cut it, using as many as possible to finally end the beast once and for all is what most Slasher hunters will do on their toughest adversaries.
The first way to end a Slasher is to remind them of their humanity/alienanity. By using one of the Slasher's still living family members or very close friends as bait, the Slasher may hesitate before killing them, and in that moment they become vulnerable to being put down for good. This can backfire if the Slasher kills the bait target instead; losing that last little attachment from the world will just make them even more dangerous.
The second method is to use a silver bullet trick. Many cultures in Garden come from worlds with rumors about special or sacred weapons; Humans have silver. Pink, eyeless Rhino people believe the Fingers of Isk'Kassa has monster-killing powers. Ratlings have poison gas from a rare type of weed in their world, and so on. These weapons ONLY work against Slashers coming from the same culture and species; so if you wanted to end a human Slasher you'd need to use a silver bullet. If this weapon hits it will only work once, and reduce the Slasher's HD by the damage dealt. If brought to zero or less HD, or if they are weakened enough to be finished off by conventional weapons, then the Slasher is finally defeated for good. It is also said that the supernatural weapons or items beneath Garden in the service tunnels may be extra effective against Slashers.
The third method involves destroying the Slasher's connection to the world. Destroying their lairs where they may store trophies or their victim's bodies, extra weapons, and create dark altars is a good way to reduce their connection to the physical world. Many Slashers leave behind their first mask and weapon in their old dwelling as a normal person, as well as sketches or diaries talking about their experiences at becoming a Slasher. Destroying these will cause the Slasher physical pain; but may draw their anger at you if it isn't enough to remove them from the world. You'll have to destroy their body again once their fetters are gone to send them to where they cannot return.
The final method involves psychic powers. Psychic probing of the Slasher's mind will only find a black hole of despair and evil that causes 1d6 psychic stress to the psychic that attempted to read the Slasher's mind; but Psychic attacks can cause damage on the Slasher's seemingly magical abilities. Extremely powerful reality warping psychics may be the only answer to the most powerful Slashers. The most famous Slasher of Garden, the Headsman, who chopped off and collected heads for his various collections was ended by a reality warper. Since the Headsman preferred to take the heads of powerful individuals, he hunted a reality warper. When detected, the reality warper simply erased the Headsman from existence, which ended his reign of terror once and for all. But there are still rumors to this day of decapitated corpses and mounted heads; some believe the Headsman managed to come back from even this and is stalking the night once again.
Roll once in each category.
Slasher's Mask – 1d12
 Made of grass and leaves, two soulless slits for the eyes.
 Gas Mask.
 Bucket. Metal shards welded on to make spiky.
 Iron tragedy mask. Random expression.
 Minimalist rubber mask. Small mouth and eye holes, black cloth around head.
 Cardboard box. The faded logo is of an offworld company.
 Leather mask made from a face of a random alien species. -1 to hit if you're of that species.
 Upper skull of a strange and feared creature from outside the city's walls.
 Wears a large hood that obscures face with supernatural darkness.
 Carved, painted wooden head that mimics a random animal or alien animal.
 Motorcycle Helmet.
 Chainmail veil.
Slasher's Lair – 1d12
 Windowless white hover-van. Never seems to drive anywhere, just shows up.
 The service tunnels beneath the city. The tunnel's inhabitants seem amused by these antics.
 Stores all of its victims' bodies in a large black dumpster until it is found.
 Old scary complex out in the wilderness made of rusty alien-world shipping containers.
 Cluttered house. Multiple booby traps and tunnels in the garbage.
 Tin shack built on the roof of a skyscraper. Uses the “out of order” elevator.
 Uses a half sunken boat in Garden's water district. Air pockets are within.
 Abandoned slaughterhouse. Bioluminescent flowers grow along the blood gutters.
 Lives in a mysterious red subway car in Garden's subway system. Common superstition means almost nobody except new immigrants use that car.
 Abandoned wing of a still active shopping mall. Entrance boarded up, little trails of money, candy, or bullets try to lure people into the dark lair.
 Lives in a back ally, sewer tunnel, or under a bridge; walls nearby are totally covered with spooky scratches from the Slasher's weapon.
 Random apartment. Strangely spares little old lady next door neighbor, and even leaves her gifts of fabric and jewelry from the murdered victims. She'd be devastated if she knew.
Slasher's Iconic Weapon – 1d12
 Big butcher's cleaver. Has several smaller ones hanging from the belt.
 Pitchfork or heavy metal rake.
 Giant power drill. Used to drill in massive screws in constructions by big aliens.
 Giant wire cutters. Used to cut the giant metal wires by Garden's power industry.
 Punching fist circular-saw blades. By squeezing the handles can make the blades spin.
 Hover Chainsaw. Same as chainsaw, but uses hover tech to make chain rotate.
 Very goddamn big kitchen knife.
 Rusted long 'toothed' saw fashioned into a sword.
 Axe of alien design. Strangely organicly shaped, but still functions perfectly well.
 Lead pipe with sharpened end. When people get stabbed, blood flows out the other end.
 Super Nailgun. Fires whatever is loaded into chamber when pressed against victims. Shards of glass, long wooden splinters, rebar pipe, etc.
 Giant sack of coins. Likes to take one coin from each victim; gets mad and brutalizes corpse if they aren't carrying any.