In this passage, Macduff is reacting to the horrible news, brought by Ross, that Macbeth, fearing the witches' prophecy to beware of Macduff, has had his family killed in his absence (hoping, of course, to kill him as well). He is struck with grief, and Malcolm, while sympathetic, tells him to "dispute it like a man," meaning that he should take revenge on Macbeth. Macduff replies that he will gain revenge in a manly way, but that he could not help but grieve as a man who has lost his family. All of the things that meant the most to him, he says, were taken from him, and he realizes, talking to himself, that they were "struck for thee," meaning that they were killed because Macbeth was after him. The fact that, as a warrior, a husband, and a father, he was in hiding and was unable to protect his family devastates him all the more. It is at this point that Macbeth has an implacable enemy in Macduff, thus, in an ironic way, helping to fulfill the witches' prophecy.