Hmm, it would depend on being a "classy" killer, or "brutal" killer.
As a classy killer, I would simply "euthanize" them gently, and place a single black feather on their chest.
As a brutal killer, I would dismember the body and place the limbs in a pattern shaped like a pentagram. To add flair, I would also light candles around the body to make it seem satanic in nature. Just for funsies
Though, a calling card would probably make it easier to track me. If I didn't want to get caught, I would change up each kill, make each more inventive than the last. Or I could also base each kill on a classic work of art.