Spock could have survivor's syndrome personified in the death of his mother. If only 10,000 did make it off that planet, a lot of them may be wondering if it was luck or logic that saved them. And why them
? They may also have a particularly tough time dealing with the mindlessness of it all. There was no logic in what Nero did. This stoic race was almost wiped out by an act of pure passion for no reason other than the blind hatred of one individual -- Spock Prime, and something that happened in another universe. Vulcans also might not be as jaded towards senseless violence as, say, humans are, and it's particularly tougher for them to cope with than they expected. After all, they do feel.
And, if Spock's still feeling that bad about it, how do you think Chekov feels? If he'd just been able to get a lock a tenth of a second sooner -- . Don't think, "Is there anything different I could've done?" hasn't run through his mind at night more than a few times?