Not sure what is going on but here is the deal:
Happens occasionally
In tactical battle, I attack and hit a unit. The red health bar above unit goes down but NOT all the way, I look at the enemy stats (lower right) and the health reads 0 (zero)/xx , where xx is the original (max) HP. During the next round of battle this unit with 0 HP attacks (does damage) and dies.
I had no poison dealing units or any damage dealing buffs (like cloak of thorns). Question: why the 1 turn delay in him dropping dead?