Hate is too obvious a direction. Better to align particular enemies with a certain ideal set that may conflict with the players. The whole "Bad guys are wiping out humanity, blah blah" tends to be done very poorly, often ending up in a black and white "They're so bad, we're so good" circle-jerk (Gears of War style). Hating an enemy we're supposed to kill isn't even necessary. I'll point out that in Shadow of the Colossus, the whole idea that we really had no reason to kill those creatures is part of what made it so compelling. Halfway through you begin to question what you're doing and the game is all the better for it. Similarly in Braid, you're never given any reasons why you're stomping on creatures and killing bosses. You just do it because that's what you do in platformers. [spoiler]The realization that you, as Tim, are the villain provides much of the magnitude in Braid's twist ending.[/spoiler] It's not as simple as making enemies we want to kill because they're bad. Sure they could do that, but that would be cheap and shallow. There are so many directions you can go with. The question is not really how to make enemies that are worth killing, but how to create conflict that is interesting and meaningful.