He is being facetious by describing himself as the 'resident bad cop', and again with the statement 'onions are my own', which is a play on words, often people write 'opinions are my own' on their Twitter profile.

Resident bad cop refers to the good cop/bad cop technique. Calling himself the resident bad cop means he always plays the tough guy role. Onions are my own is a play on the disclaimer “opinions are my own” which means you are representing only yourself and not your employer or any other group.

"bad cop" is an expression that derives from the phrase "good cop / bad cop". In that scenario, the "good" cop (cop means police office) acts friendly towards the suspect and the "bad" cop is aggressive towards the suspect. The idea is that the suspect will see the good cop as his friend and be more agreeable. By "resident" he means that he is the bad cop in that area.

He is being facetious by describing himself as the 'resident bad cop', and again with the statement 'onions are my own', which is a play on words, often people write 'opinions are my own' on their Twitter profile.