Antonin, unerringly, had put two and two together and made "no one in power will hassle me for killing things if they're zombies." He can't be in Severus' house all the time, and he is really very good at killing things. The way he looks at it, this is almost like community service.

(The analogy would probably work better if he were more careful about making sure the people he killed were actual zombies and not just covered in blood because they were running away. You can't make an omelet without killing a few innocent bystanders.)

And if he sees someone who is clearly alive, he might even help them. For the novelty value.