I'm scared of him. That man is her killer.
I'm scared of him. That's the man who killed her.
They can be used interchangeably, but "killer" means "one who kills" and has an implication for habitual action. Why are these sentences basically equivalents? Why is "that's the man who killed her" closer to what we want to convey rather than "that's the man who kills her" when we say "that man is her killer"? I don't see how one sentence like "that man is her killer" can imply both habitual and non-habitual actions. If you say "That man is the bathroom's cleaner" then it's implied that he cleans the bathroom habitually, but if you say "That man is the company's founder" then you don't imply he habitually founds the company where he constantly tears it down and founds it again.