Malignant cells are duplicated before the attack damage is done. So say you only have 5 Malignant cells on your field, and the enemy has 10 hp left. Once you end the turn, all your malignant cells will duplicate, BEFORE damage is applied, so you will have 10 cells and kill your enemy.
I am guessing this is a bug, as it says it is done at the END of every turn.