I'm not sure there is one. For example, while "homicide" has "murderer," "patricide" and "regicide" are used for both the perpetrator and the act itself.

This is really bothering me.

What do you call a person that commits homicide? A murderer. I can't even break down the word genocide comparing it to homicide to get a title.

The perpetrators of the Rwandan Genocide were referred to even in English-speaking media using the French loanword génocidaire. An English calque of that word would be something like genocidor or genocidary, neither of which are actually real words in the English language afaik.