In this scene, Richard III is contemplating murdering two princes who are his nephews and marrying their sister to secure his crown and throne. Obviously both such actions are evil, but ‘So far in blood’ refers to the fact that he has already murdered other people to get the throne, so ‘But I am in’ is his statement that he is already committed to this path and is prepared to do whatever it takes. Therefore, he is willing to perform multiple acts of evil, or pile sin upon sin (a bible verse from Isaiah 30:1). Shakespeare’s choice of the verb ‘pluck’ in place of ‘pile’ may also be a reference to the bible verse (Matthew 5:29) where if someone’s eye causes him or her to sin, it should be plucked out, so in Richard III, instead of plucking out sin, sin will ‘pluck on’ sin. Together, the metaphors convey that Richard's evil plot is getting bigger and deeper, because he will do anything to become and stay the king.
this quote means that king richard has slaughtered his wife and has gotten himself so far deep in her blood that he feels guilty for feeling happiness in killing. He understands the chain reaction of sin, that it will pluck more sin in the future, such as killing another character from the play.
i hope this helps