Look at all the good things Tyrion did before he killed Tywin and became this miserable, alcoholic bag of cynicism:
1) Not raping Sansa, when doing so would make him Lord of Winterfell.
2) Sending all the members of the City Watch who participated in the atrocity of killing Robert's bastards, and specifically arranging the death of the gold cloak who killed the baby, Allar Deem.
3) His disgust at Cercei's vicious political tactics
4) Helping Bran with his saddle.
5) "And why would I ever need your Allar Deem, Lord Slynt? Tyrion thought. I have a hundred of my own. He wanted to laugh; he wanted to weep; most of all, he wanted Shae." He is disturbed by the evil that surrounds him, and to cope with it, he uses humor, especially cynical humor.
6) His kindness towards Tysha, and his hatred for his father, which is due to what Tywin did to her.
7) Even after Tywin's death, he shows kindness to Penny, and feels sorry for her ("She deserves better than a pig")
I'd say Tyrion began as a good, compassionate character, but all the cruelty that has surrounded him since his birth has slowly turned him into a dark and neutral character, whose only purpose is vengeance and hedonism. To cope he has developed this cruel of sense of humor that we see especially at A Dance with Dragons.
Although not evil, he has become apathetic, cynical and VERY misanthropic, but deep down I believe he is a genuinely good person, although that side of his personality may die by the end of the series, or surface.
Edited by Panos Targaryen, 28 June 2013 - 04:05 PM.