The buff that makes zerg get +1/+1 make the guy stay alive but he should be dead. It goes to zero then back to 1 health after the minions die at the same time
Nothing in Hearthstone happens “at the same time”. Minions that have lethal damage are removed from the board one-by-one.
When the Infestor is removed from the board, its deathrattle triggers, granting +1/+1 to the damaged zerg minions remaining on the board.
If this brings their health above zero, they are no longer lethally damaged, so they don’t die.
This is not anything new. If some effect casts a bunch of random spells, it can play Flamestrike and then Circle of Healing, which deals lethal damage to your enemy’s minions and then immediately heals them. The game has always worked this way.
won’t be a problem for a while in standard since that buff is getting nerfed next week and will no longer give health