These lines occur in Act III of Shakespeare's Macbeth. In Act II, Macbeth killed King Duncan. Now that Macbeth has succeeded in taking the throne, he is suffering from paranoia. The cause of his paranoia is the part of the witches' prophecy that predicted that one of Banquo’s line would someday be king. This leads Macbeth to feel that, as long as Banquo and his son are alive, he is not safe. 

The quote reveals that being king is not enough for Macbeth, he must be king and be “safe.” Banquo is something to be “feared.”

As we can see, one act of violence leads to another.