We eat meat to. It's be different if they ate humans, but if they just ate steaks and pork chops it wouldn't be any different to things here. You'd think they be scared coming toward a species that enslaves and force breeds others on farms just to eat them(a.K.A. Humans)
As a vegetarian, I don't think that any person on Earth that eats meat is evil. How would an alien be any different? Being a vegetarian/vegan is a lifestyle, and there are more reasons to choose it besides "it's cruel to the animals." Unless the carnivorous alien was trying to eat ME, I wouldn't call it evil, because it's just doing what the majority of people on Earth do.
Humans, are naturally herbivores. Comparing ourselves to carnivores on earth shows us that we don’t have claws, sharp teeth, short intestines, etc. I don’t think wolves or lions are evil because they evolved to eat meat, that’s just who they are. Why should I think aliens are evil for living on a diet that their bodies need to survive?