A disbeliever and a believer who killed him will never be gathered together in Hell.
Sahih Muslim Book 33, Hadith 191
← Previous Hadith God laughs at the two men one of whom kills the other; both of them will enter Paradise. They (the Companions) said: How, Messenger of Allah? He said: One is slain (in the way of Allah) and enters Paradise. Then God forgives the other and guides him to Islam; then he fights in the way of Allah and dies a martyr.

Sahih Muslim Book 33, Hadith 190 Next Hadith → No two such persons shall be together in Hell as if one of them is such that his presence hurts the other. It was asked: Messenger of Allah, who are they? He said: A believer who killed a disbeliever and (then) kept to the right path."

Sahih Muslim Book 33, Hadith 192