Do you say "matar (conj.) + a + noun" or just "matar (conj.) + noun"?
For example.. Él mata mi gato. O Él mata a mi gato. Thanks!
You will find many cases contrary to this but the rule is:
In Spanish, direct objects that are both human and specific require a special marker (the preposition a "to").
Pedro besó a Lucía. = Peter kissed Lucy. (Literally, "Peter kissed to Lucy")
Non-animate direct objects do not usually allow this marker, even if they are specific:
Pedro besó el retrato. = Peter kissed the picture.
Yet, some animate objects that are specific can optionally bear the marker:
Differential object marking (DOM) is a linguistic phenomenon that is present in more than 300 languages apart from Spanish.
If your attitude towards the cat is very personal (like it was your friend), then I think saying "Él mató a mi gato" would sound fine. But if you are telling someone that he "just" killed a cat (any cat, one you don't really care about) you would say: "Él mató un gato." But let's wait for other opinions.