They won't kill him off, he is too loved. But he most certainly needs to be rested.
You say that but they do have a habit of killing people in these Marvel based films.
If you think of the first three Spiderman films, Goblin...dead octopus...dead... Sandman....dead ( I think, it's been a while since i seen it)
Captain America - Red Skull.... dead
Iron Man... Jeff Bridges character... dead
Iron Man 2... Whiplash.... dead
X-Men jean Grey.... dead Cyclops... dead Professor X.... dead (or is he?)
Wolverine.... Sabretooth... dead (if i remember correctly)
They have no problems with killing characters in these films. most likely as they have such a huge gene pool to pick from they don't need to worry abiut re-using them all the time.
It's already been announced that Loki won't be in Avengers 2, so they don't need to keep him alive for that.