"Where did you get those gloves? I thought we left them in the garage," Malachi inquired. He made sure to stand a few metres away from his wife as he spoke. Not because he was afraid of infecting her with the zombie blood on him, but because he was scared of her impression of him.
Looking at how even Mia seemed to be very wide awake at the moment, Malachi figured that together with his son, everyone in his family should have seen what he did to that zombie or at least a part of it, and any part they saw was still enough explanation of what action he was taking.
It was when Malachi saw his wife that he figured that sooner or later, the moment he started killing zombies for their crystal hearts, they were going to know of his action.