As @Dizb already mentioned, Hellborns can reproduce. They are born and live in Hell from the start. For example, in the original show Millie was shown to be pregnant. Sinners, who arrive in Hell because of their sins, cannot reproduce. That is part of their hellish punishment.
As for I.M.P killing inside Hell — yes, that is unusual, and that is the point! Later in the visual novel this will make sense (maybe even a lot of sense!) 😊
Hope this answers!