Don't even bother.

That person is so obviously full of shit, picking the most obvious and easiest strawman to beat on.

I am vegetarian and know many vegans.
None would even think about torturing our pets to death with food that is not edible for the specific species.

On the other hands I have heard multiple omnivores tell stories about how they "know" crazy vegans that would do shit like that.

There are some crazies (like in any big group of people) and I can't know if OP accidentally ran into one, but the way they generalize makes it clear to me that they are the one with issues, not every run of the mill vegan.

Vegans usually care a great deal about animals, including carnivorous animals. So fuck them right back for spewing their indiscriminate hatred.