Fix your lenses forensics would've told you Kendrick had killed it

from Kendrick Lamar (Ft. Dr. Dre) – Compton Lyrics on Rap Genius

Meaning

He’s suggesting the listener pay closer attention when listening (metaphorically fixing their lenses-improving their sight of his overall vision) because in doing so it would be obvious the amount of skill(killing) he applied to the track.
The fact that he says forensics(crimelab) would’ve also told you had you not seen it before suggests he left the track in a state similar to that of a decomposing body

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