If I saw my best friend killing my boyfriend/fiance/husband... Jeez... I seriously have no clue what I would do. First I would probably ask him (my best friend is a boy) what the hell he was thinking, and why he did it. Then I would probably call the cops. I do not think I would act violently towards my best friend. I would just want to know what was going through his mind to make him kill my boyfriend/fiance/husband.