I've encountered this many times, and while I do not automatically judge one with a more 'tragic' back-story, in fear of being judged merely because so many see this sort of thing to be negative, even if the character does not act in the manner you might've mentioned (I've made sure to steer clear of mere mentioning of it, you know?), and this observation and what sort of backrounds and how a character goes around on it pleases me very much!
Very little users do not look at the physiological effect that the dwindling on such subjects (or how it is held in the view of the character); they are typical-acting, perhaps even excited, however the manner they act when they think of the subject or talk about it differs very much from their actual personality. If one is to dwindle on such a subject, and if the subject is to leave such a dreadful scar, they would, as you say; not act normally. Nevertheless one of the things that I adore is when things that happen to character and the manner they view it effects their personality and over-all mental state correctly.
Examples would be in The Hunger Games, where Katniss Everdeen reacts in a somewhat logical manner to what she has gone through. At the age of sixteen, she is forced into an arena to kill others; almost commits suicide after learning that she would have to kill who she was with the entire time, was forced into the games once again, where the people she knew before her first set of the games seemed a bit more, 'alien'; Gale, mostly speaking. She went practically insane, and in the prologue she even mentioned that she feared having children, in the cases that they may be taken away like others' had been.
Another would be Fyodor Dostoevsky's Crime and Punishment. Rodion Romanovich Raskolnikov killed a pawnbroker and (forcefully) her younger sister in order to test a theory. This back-fires, and along with trials that test the true power his mental state holds, he eventually breaks. However, it is mentioned before that he came from a poor family with no father; he reacted exceptionally well, but how he treats himself (barely eating, etc. . ) just from the start along with flashbacks of what might've gone on in the past contribute further to his insanity.
I get what your saying, of course; sadder backstories are fine, oh yes quite fine, but what of the manner in which they are portrayed? What about the logic behind the character's behaviour?