But Tyrion IS a man devastated by pain, his whole family have abandoned him, no its worse they have turned on him, he has received little to no love his whole life, the way they all jumped straight to accusing him when Joffrey died must have been devastating, he's grown up knowing his father despises him, his sister is repulsed by him and unbeknown to him believes he will kill her and her children, and then the only member of the family who was ever kind to him admits to having done him a great wrong in lying about the only women who has ever loved him for him.
He gets darker because of what he's feeling about himself & his family. He kills Shea without even thinking for one minute of she deserves it, if she had any actual power over appearing in that court room or what she said whilst there, she says Cersei forced her to speak out against him, well we all know that that is VERY plausible, just look at Cersei's track record. And as I've already mentioned do we really think she had any choice about ending up in Tywins bed?
The point when he begins his descent is with her murder, there is a very clear progression to that point too. He is angry and hurt and years of baggage are spilling out into his mind, which continues to nag at him as we follow him across Essos. When we see him rape the sex slave in Volantis he's reached a complete low. One which he is beginning to climb out of once he has experienced slavery, and how those who are not in his privileged position are living completely at the whim of those in charge.
This is Tyrion's journey. I feel he has to be shown as getting dark in order for us to rejoice when he comes back, I'm not a huge Tyrion fan he is not a favourite of mine, but I do like him. I don't judge characters though. But I think D & D will be making a mistake if they don't show him unravelling.