Besides, even if he never intended to actually harm Thomas, he still had absolutely no issue convincing him that he would. They made it clear that Harvey had an edge to him, particularly when it concerned his loved ones. It's all well and good to be righteous, until someone you care about gets threatened. Then you're just a man trying to protect someone you love.
He wanted to keep Gordon's unit at arm's length, but ultimately put his faith in Gordon. And how is that trust rewarded? One of Gordon's officers gets Rachel killed, and the other gets Harvey maimed. For the mob.
Harvey spends the entire film trying to do things by the book. Even putting himself in the line of fire to get the Joker off the streets. But guess what? Justice doesn't get its day. The bad guys still win, and the only thing Harvey gets to show for it is a complete heartbreaking loss.
Disillusionment, betrayal, heartbreak, anger, shock. The guy had nothing to live for anymore, and here comes a madman who simply aks the question, "You see? What's the point?" And yeah. In Harvey's mind, what was the point anymore?