Macbeth utters these lines at the end of a long and emotionally charged scene. The scene begins with Macbeth welcoming his guests to "a great feast," as Lady Macbeth calls it, to celebrate Macbeth becoming king.
Macbeth sent murderers to kill Banquo and his son, Fleance, while they were riding in a park near Macbeth's castle. One of the murderers appears at the beginning of the feast to tell Macbeth that, although Banquo has been killed, Fleance escaped. Macbeth is happy to know that Banquo is dead, and although he's still concerned about Fleance, he thinks Fleance is too young to challenge his throne:
MACBETH: . . . the worm that's fled
Hath nature that in time will venom breed,
No teeth for the present. (3.4.32–34)
Macbeth rejoins the feast, gives praise to Banquo, and is then appalled to see the ghost of Banquo sitting in his chair at the banquet table. Lady Macbeth tries to carry on as if nothing is amiss while Macbeth accuses his guests of playing a trick on him. He then talks to the ghost while his guests watch him in amazement.
Just when Lady Macbeth gets Macbeth settled down and the feast returns somewhat to normal—Macbeth even gives a toast to Banquo—the ghost appears again, and Lady Macbeth is forced by Macbeth's disturbed behavior to ask everyone to leave the banquet.
Left alone with Lady Macbeth, Macbeth begins to ramble, almost incoherently.
MACBETH: It will have blood: they say blood will have blood.
Stones have been known to move and trees to speak;
Augures and understood relations have
By maggot pies and choughs and rooks brought forth
The secret'st man of blood. (3.4.149–153)
For no reason, Macbeth asks Lady Macbeth what time it is. Then he wants to know why Macduff didn't come to the feast. He talks about having a spy in Macduff's household and tells Lady Macbeth that he's going to visit the witches to find out what is going to happen to him.
MACBETH: And betimes I will, to the weird sisters.
More shall they speak; for now I am bent to know,
By the worst means, the worst. (3.4.162–164)
Macbeth then says the lines which contain the referenced quote. It's important to look at this quote (as with any other quote) in context. Macbeth first says that from now on, he's only going to do what's in his own interest and for his own good, and nothing is going to stand in his way.
MACBETH: For mine own good
All causes shall give way.
Then he rationalizes, as other Educators have discussed, that he's gone so far down this road of murder in order to acquire and keep his throne—he's killed Duncan and Duncan's two guards, and had Banquo killed, so far—that he can't turn back now, even if he wanted to stop all the killing.
MACBETH: I am in blood
Stepp'd in so far that, should I wade no more,
Returning were as tedious as go o'er.
Then he tells Lady Macbeth (and us) that he has other ideas, "strange things," in mind along this same murderous road, but he can't talk about them until they've been carried out.
MACBETH: Strange things I have in head that will to hand,
Which must be acted ere they may be scann'd. (4.3.163–169)
Macbeth adds one more line that is even more ominous and disturbing than anything else he's said in the scene:
MACBETH: We are yet but young in deed. (4.3.173)
In other words, Macbeth is just getting started.