Malachi's heart raced as he saw the actions David was taking.
They had to leave the area quickly because of the zombies that were starting to come from all sides and would eventually surround them.
Individually, these zombies did not pose a threat, but it was likely not the same for his wife and child. Not that he also wanted them to fight in the first place.
And from what Anna and Mia had recalled for him about the event that happened on the highway when the cracks in the sky appeared, Malachi knew that one bite was all it took for everything to fall into ruins.
Malachi would absolutely not put his family at that risk if he had the choice not to.
While Malachi was angry at David's action at the moment, inside, he could also understand him.
However, understanding did not equal to appreciating or supporting his actions.
When he saw his son take out the monster quickly, Malachi was shocked but at the same time, relieved.