Why does the quote "there's blood upon thy face" cause tension?It's from act 3, scene 4 - Macbeth says this.
The sight of blood is always vivid and shocking. The tension it creates adds to the suspense that the audience feels and marks the dastardly deed with an actual visual sign or image, bringing the reality of what has happened home to the audience.
When the murderer responds with an answer as to the origin of the blood being Banquo's, Macbeth responds that it is better on the murderer's face than still in Banquo's body (ie than that the victim is still alive.)
Macbeth is no longer just talking,but doing! The thought has become the deed and a 'venial' sin can become a 'mortal' one.
Just to make sure (as it's possible that a victim who has been injured only,not killed, would be bleeding) Macbeth makes sure to ask whether Banquo is actually dead. The murderer tells him that he cut the victim's throat - another vivid and ghastly image, adding to the previous layers of blood imagery and heightening the tension - we know that the other target got away and are wondering what Macbeth's reaction will be when he hears about that.
You can see how the tension builds, as you study the text, line by line: