Well if you want to get technical about it - wishing someone to die in their sleep is the most peaceful way they could hope your end to come.
If you want to get even more
technical about it, the concept of painless death during sleep is a large myth that's widely believed. While it's true that one can indeed pass peacefully (though a nightmare before one's final moments may happen), many of the ways that one dies while sleeping are in fact painful and do cause the person to wake up before their final moments. Therefore, not only are you in pain and obviously dying, but you usually can't get help, are alone or too weak to wake someone next to you, and are doomed. It's actually a painful, scary, and possibly lonely way to go.
It's just like how lethal injection is incredibly painful (perhaps even moreso than the electric chair for some) but doesn't look it, making people believe it isn't. There are exceptions, just as there are with the circumstances under which one can die in their sleep, but those are indeed the exceptions and not the norm.
How's that for a bright spot in this thread? :D
I hope I didn't make all of the people who shouted out to me suddenly hate me, or step away in fright. :D
[Kohan - MNK of Pandemonium]
[Shells - TheShadows, Delevel]
BM: Y'know, I think my favorite thing about being a hero of destiny is that it gives you all kinds of narrative justification to just slay any ol' jerk who gets in your way.
Thief: Here, here!
RM: That IS a nice perk.
Fighter: So's the free swords! ... I will name them Stabby and Slashy. FFXI Bandwidth, Graphics & CPU Optimization Guide