I'm murdering this beat as if nobody was watching me
He ‘murders’ the beat, which means to put a great song down on it.
He does this like nobody is watching him, much like a real murderer would do, so that there are no witnesses to the crime.
This could also refer to the fact that he acts like he doesn’t care what others think of him, so he literally raps like nobody is watching him.
To help improve the meaning of these lyrics, visit “For Your Love (Freestyle)” by Childish Gambino Lyrics and leave a comment on the lyrics box