There is not yet so ugly a fiend of hell As thou shalt be What does this quote mean?

This is a line from Shakespeare's play The Life and Death King John, Act IV, Scene iii; the character Bastard is speaking to Hubert, accusing him of killing Arthur (who died jumping from the castle wall). The full quote is as follows:

Ha! I'll tell thee what;

Thou'rt damn'd as black-nay, nothing is so black;

Thou art more deep damn'd than Prince Lucifer:

There is not yet so ugly a fiend of hell

As thou shalt be, if thou didst kill this child.

Hubert is declaring his innocence, and Bastard is telling him that if he is in fact guilty, there is no being more evil than the one who did kill Arthur.