Does Macduff kill Macbeth?
Macduff does indeed kill Macbeth in act 5, scene 8. Macbeth has developed a false sense of invincibility, having been told by the witches that no man "of woman born" can kill him, but Macduff advises him that he was in fact "untimely ripped" from his mother's womb—that is, he was born by Caesarean section, which is presented as a loophole in the context of the play.
The actual death of Macbeth does not happen on the stage itself, as he and Macduff exit fighting, but it is very clear from implication that Macduff is the instrument of Macbeth's downfall. Macduff reenters the stage carrying "the usurper's cursed head" in his hands, so there is no doubt that Macbeth has indeed been killed during the offstage fracas.