What are the circumstances in which these lines are spoken?
O, what a deal of scorn looks so beautiful
In the contempt and anger of his lip.
A murd'rous guilt shows not itself more soon
Than love that would seemhid: love's night noon.
These lines are from Twelfth Night Act III Sc. 1. These lines are spoken Aside by Olivia whom Duke Orsino is in love with. Viola is now disguised as a young man and has assumed the name of Cesario. Orsino has assigned the task of wooing and winning Olivia for himself to Cesario. By a strange quirk of circumstance Olivia falls passionately in love with Cesario (Viola) !!
So, in Act III Sc. 1 Cesario arrives at Olivia's house to woo Olivia on Orsino's behalf. This only kindles the passionate love of Olivia for Cesario (Viola). Viola of course has no other option but to angrily reject Olivia's advances even at the same time maintaining the secrecy of her true identity. This annoys Olivia and she angrily dismisses Viola from her presence,
There lies your way, due-west.
Viola leaves Olivia's house but not before expressing her displeasure at Olivia's rude conduct. It is then that Olivia who is captivated by Viola's angry looks speaks these lines in an Aside:
(aside) Oh, what a deal of scorn looks beautiful In the contempt and anger of his lip! A murderous guilt shows not itself more soon Than love that would seem hid.
Olivia speaks these lines to herself, meaning that even when Viola (Cesario) is angry and irritated she appears very attractive and desirable. She then remarks that it is easier for a murderer to hide his guilt than for a person to suppress and disguise his love when he is in love.