Personally, I feel there is one moment where Tony shows just how ugly he is inside that stands above all. It's during season 5 when Janice takes anger management therapy, and he sees how well she seems to be doing, and he can't stand it, so he provokes her by bringing up her estranged sun.
Not even killing Chris compares, because at least there was some logical reasons for that(he was a liability). But seeing your sister, who you know had the same fukked up childhood and mother as you, finally starting to put her life on tract and become a better person, so out of jealousy you just ruin it and laugh in her face. That's like... so petty, and selfish.
Heisenberg has done some pretty terrible things, but I feel that even in his darkest hour Heisenberg never sank that low. Except maybe when he told Jesse he watched Jane die. But even if that was a deliberate twist of the knife, at least at that point he and Jesse were at each other's throats. There was no love at that point.
What Tony pulled was just a ruthless, calculated attack on a person he supposedly loved, meant to break her down and bring her back into misery(the same as him) just out of pure spite.