If you read Cain's lore on Diablo3.com, apparently he, at one point, was an adventurer/warrior much like the heroes are/were. But now he's old, and I see your point. I'd imagine the reason he wasn't killed in Tristram is because since he's a Horadrim, he would have an excellent amount of knowledge that the Three could use against the High Heavens. And the reason he seems to always be in "the right place" is because his quest and yours is to destroy the Prime Evils. He would logically, being the scholar he is, want to go to the places where the Prime Evils are, I.E. where he can find out the most about them. That's my take on it.
true... AND he probably wasnt killed in tristram because he would've known something about Baal And Tal Rasha maybe
I don't have much lore background, just what I remember from watching my brother beat D1 when I was seven, and my experiences beating D2 and LOD several times by myself. No books. So I'm not really making this "theory" based off of lore. It's more of my ideal "oh, that was unexpected" plot twist.
However, after reading what Raven Master posted, he seemed to be making some found points. Cain is just always in the right place at the right time without getting hurt in the process while we die 20 times AND WE'RE THE ONES WITH ARMOR AND SOWRDS AND MAGIC. He's just a frail old man. Just saying...
maybe he uses some horadric magic and tricks up his sleeve
umm i gotta disagree on that plot twist ... cmon how many plot twists we have here?? tyrael cain and many more. I dont think they can pull it off and i would hate to have to kill that old man... since he identifies items for free
true... AND he probably wasnt killed in tristram because he would've known something about Baal And Tal Rasha maybe
maybe he uses some horadric magic and tricks up his sleeve