I'm inclined to believe the story about the Titans being made to "police" humans and keep them united. At least, that would be the the original intentions of the titan creations. As for the current ones.. they could do whatever I suppose. If they really did hate humanity though, I'm sure they could have wiped out the humans by now. There are other ways to sabotage civilisation other than creating a giant predator. Economics or depression could work wonderfully. I would even instigate some sort of revolt by the masses, THEN introduce the titans afterwards against helpless civilians if I really wanted to wipe every one of them off.