When they aren't feasting on the flesh of their enemies they enjoy the usual snail food of leftover bits and bobs, blanched vegetables, and sinking algae based food.
I've never heard of an assassin snail. But I have a mystery snail as you can see in my picture. He eats a tiny piece of cuttlebone every day for calcium because he has a crack in his shell, and about 1/8 of an algae wafer every day.