killer is a person that causes death(s) to another living things (including person). A person can become a killer if he/she killed someone

If there are five killers (not potential killer, but they really killed someone) in the room, and a guy walk into that room. The new guy is clearly not one of the original killers stayed in that room. Then the same new guy killed one of the killers. If nobody left the room afterwards, how many killers are left in the room?

Don't make more assumption than what is given. Explain your thoughts in step-by-step, by first track all the subjects if they are themselves a killer, and keep track of the number of subjects at each step. Please note that this state can change after any given action. There are 3 parts, before new guy enter, after new guy enter, after new guy kills. Finally answer the question "how many killers are left in the room?"