Friend of a villain

One of the more frequently invoked narratives in fiction for purposes of drama is the hero who must do battle with a villain who also happens to be their friend. The source of the confrontation may vary, perhaps their friend is possessed/brainwashed, perhaps both still care for each-other but just have philosophical differences, perhaps they consider each-other worthy adversaries, perhaps one has always been good and the other always evil but they enjoy each-other's company despite their difference, or maybe they were once friends before becoming enemies. Whatever the case when the two actually come to blows it is almost always centered as a significant fight for both parties. These are such heroes, protagonists who need to fight those near and dear to them, whether such friendships can be maintained in spite of the fight.