It isn't bad to look at or want another cat any more than it's bad to look at or want another child. It means you have a big heart, not that you're a bad person. Don't allow superstition to prevent you from acting on giving another cat a much needed home just because you're feeling like you may cause misfortune to the cat/s you do have. You're being prompted by your spirit to reach out and this is only a good thing. Relax.