It was just a single scene and it had purpose, nut just for lolz: It shows how delusional Ivan is, that he is unable to realize when something bad happens. This has nothing to do with hating cats whatsoever. Gunnar loved his cat. That's the point. The cat got killed accidentally and it was supposed to be shocking in order to show how dysfunctional Ivan is. Because people like you are not shocked enough if blackbirds are killed (why are you concerned about a single cat but not about all those birds getting killed?).
This film should have a different warning that you proposed: "Stay away if you don't understand how black humour works"
What a ridiculous protest. It is very clear that the cat's death is "gratuitous" and reflects the insanity of a supporting character. It's a wonderful movie in which occasional cruelty is used to advocate by contrast the most humane values. If you can't get that, you need to stick to Disney movies.