Slade killing Moira Queen - in addition to coming completely out of nowhere - has, in my mind, catapulted Deathstroke into the upper echelons of great villainy. The fact that he did this and everything else as part of his revenge against Ollie just makes it greater, adds more depth and emotion to it. He's not taking a shit all over Oliver Queen's life because he's a supervillain, he's doing it for personal reasons. I think it goes beyond him blaming Ollie for Shado's death. I think he blames him for his own continued existence; that Ollie injected him with the Mirakuru instead of letting him die, because in his mind, at least he would have been with Shado after she was murdered. Or maybe it's self-loathing that he has these abilities granted by the Mirakuru and he couldn't use them to save the woman he loved.
But yeah, systematically destroying a man's life one piece at a time and not killing him? That's some next level villainy right there.