Actually, in that case, if they had free will only in regards to whether or not to resist the haste spell, they'd be stupid *to* resist it.
Let's say that the two monsters are alike in every way (lifted straight from the Monster Manual). If we're assuming that Monster A and Monster B are going to fight each other every round that they're confused, then there's a good chance that one or the other will die. It all comes down to luck - each one has a 50% chance of being the survivor.
Now, you're casting Haste on both of them. If they have enough control left to know what's happening and try to resist it, then they're faced with a few different scenarios.
1) Both attempt to resist. Again, we're assuming they're the same, so they have the same chance of resisting. Regardless, we assume that they're either both hasted or both manage to resist, which doesn't change our overall scenario.
2) Monster A attempts to resist, Monster B does not (or vice versa). In this case, we'll assume that Monster A does, in fact, resist (as otherwise, there's no change to our scenario). Monster B is now in a greatly improved position, and his odds of surviving increase dramatically, since he gets all the benefits of Haste and Monster A does not.
3) Neither attempts to resist. Again, they're even, since they're both hasted.
Now, obviously, we would need to determine how much free will they have and how much thought they're capable of while confused. However, every logical solution by a creature motivated by survival (which fits both a confused creature that is striking out against anything near it, and more rational beings) would be to allow the haste spell. If one does choose to not resist, he must trust the other to not resist, which, even if we make it completely random, will only happen 50% of the time. If it does happen, then they each stand a better shot of surviving until the confusion spell wears off. If it does not happen, then the one who does resist is much more likely to die.
Basically, the only reason someone would try to resist the haste would be if they completely trusted the other person to also resist. And, if you're so confused that you're attacking said person in the first place, I don't think your mind is clear enough to make that decision. At that point, it would really be more of a primal, survival-first thing, which says "Haste me before the other guy kills me!"
Now, this does assume that they can differentiate between a beneficial and a harmful spell. An easy way to do it might be things like Sense Motive or Spellcraft, but, again, mind isn't all that clear. Could also just have them determine randomly if they want to resist it. Or, in the case of a confused PC... just tell them that a spell is being cast on them. Do *not* tell them what it is.
