They believe a strong elemental or magical strike will reset their body's flow of energy and help cure them of a malady. There is a 50/50 chance this is true and it actually works!
 It hit in the right time and in the right way, will put them in a 'death like state' that they can be woken up from with a special process. They will use this to fake their own death to (1d6);
- Get away from the nagging wife and kids. They'll come back in a couple weeks.
- See how people get along when they're gone and see if anyone even cares
- Tax benefits- They're about to come into some wealth- best to collect when deceased.
- Become a martyr for a religion they aren't really super serious about following
- See what's on the other side
- Avoid an assassin or bounty hunter trying to kill them for real. (This one is stolen from Oblivion)
 The weapon is cursed and fated to take the life of their whole bloodline. If they can get you to hit them with a healer nearby or make sure you do it softly, eventually it will do enough points of damage to kill them all- but survivable. That will wiggle out of the curse on a loophole, right?
 The weapon's magic effects actually grants a boon to anyone who "is struck by this weapon and lives to tell the tale". Having a nice, safe way to get that benefit is worth the trouble- just don't let any of your enemies live after getting hit by this!
 The quest giver has magic blood which actually extinguishes magic items. They don't think anyone should have this one; even if you kill them afterwards for lying to you you'll still just have a normal hunk of metal instead of some earth shattering cool magic sword. It's for the best.
 They're writing a novel and really want to understand what a +2 Vorpal Flaming Mace of Shattering feels like for their character. They need that authenticity.
 They're a Cleric or a Whitemage and need to test out a new healing spell.
 They're an Alchemist and prepared a secret "blade lotion" to protect themselves against this weapon and want you to hit them in public to advertise it. 1 in 4 chance the lotion doesn't work.
 The blood or wound from the blow will make a cool portal that leads to a demon's realm- the same demon who has been haunting them. They didn't tell you this to begin with; because now if you don't go into that portal and kill that demon it will get out and attack everybody.
 Need to talk to an deceased ancestor whose soul is trapped in the blade or in an afterlife where only people killed with X weapon go to. As for how to get back? They didn't think that far ahead.
 Made a stupid bet with another blowhard they know on who is tougher. This magic item is +1 stronger then the one their rival already survived getting hit with.
 They're honestly just straight up done with life.