Driven by survival instinct, cats have virtually zero tolerance for unfamiliar members of their own species. This is especially true for two unneutered male cats, who are often extremely hostile towards each other and will often fight tooth and nail for mating rights. 
Cats are very territorial, and if another animal suddenly appears in their territory, this uninvited guest will steal resources from the cat's territory, such as food, companions, or even the owner's love.
Therefore, if you already have a cat in your home and are introducing another cat, you must first isolate them. Only after confirming that neither cat has any malicious intent should you gradually introduce them to each other.
