Animate and Create Undead are similar spells. In the first, you animate a corpse that has been dead a day or less and it's completely under your command (and count as Class I for Turn Undead and don't get RR). The base for the second is that they can have been dead for up to a week, and unless controlled, even Class I undead attack the nearest living thing. Because Skeletons and Zombies are classified as nearly mindless, I don't really understand why they would, unless it's a nod to zombie movies, it's somehow instinctual ("I should not be"?), or I missed something about Create Undead creating an unnatural evil life force. For Class II and above, I'm somewhat fine with this, it really depends on the monster.
Maybe I'll house rule that Skeletons and Zombies don't just blindly attack things. I'll read what your opinions are before I make that call though.