In many cases hatred would peter out into indifference

Perhaps, but this is not likely to happen in the middle of a battle where you're trying to kill each other. And even if you felt indifference, you would still have to think of trying to cast Avada Kedavra from your indifference, not from your hate, which is how you learned to cast AK in the first place and never questioned. You would have to force a new mindset of calm emptiness upon yourself, which would take practice. Even the worst Death Eaters are not likely to have taken an analytical approach to battle, realized the possibility, and then practiced killing people in their spare time with indifference to make sure it was reliable in the (other guy's) heat of the moment.