I attacked and killed one of the unrecruited heroes (one of the spider riders) in my game but it was still there with one hit point afterwards. I killed it twice more on the same turn with the same result. After the end-turn it healed to 8 points and I killed it 3 more times. I parked a few of my champions next to it and killed it every turn for a while just for the XP.

The unit had regeneration on it so I'm wondering if it is being killed and then regen gives it one hitpoint even after it's dead and the battle's over, bringing it back to life again.