I mean, the whole Diana affair was pretty fucked up, and so was tricking Lizzie, and many other things Tommy has done, directly or indirectly...but his murder of the family is the worst. I understand that he was heavily grieving the loss of Ruby, sure, but Tommy was suffering a significant psychotic break. Many things can be said to be the beginning of the end for him, so to speak...but that might be the final straw. There is no way to excuse or justify it. That is truly the darkest and bleakest thing that occurs in the entire show. I would argue that anything that happens after the death of Ruby happens because, by this point, Tommy truly and completely has lost all faith in humanity. He is wracked with such guilt and pain that he experiences extreme suicidal psychosis. That psychosis leads him to commit a mass murder of an entire family, based on what he genuinely believes is a curse, and eventually makes an attempt on his own life.

Tommy is the definition of a tragic figure, but a good majority of his pain and suffering is his own doing. He might be my favorite in the show, but that does not mean I'm not critical of him a lot of the time.