If you’ve taken in a cat you have committed yourself to its care. It’s not a matter of the moral agent being a killer as the combination of being a moral agent and not an obligate carnivore. You can choose not to consume animals because you don’t need to. If you choose for your cat to not consume animals, you are killing the cat. But this is not a huge factor in the real world when it comes to the meat industry. Cat food is not nearly its main driver.

As for your alien hypothetical: Whether they should be “allowed” is kind of an unclear question. Who is allowing or disallowing this action? I think humans would be justified in fighting back to prevent our own demise. I think it would be ethical on the part of the aliens to strive towards a solution for their diet that doesn’t involve killing or harming sentient beings