What does 'do the worst' mean? Does he mean he will behave badly like a bad boy?

Native speaker’s answer
Rebecca
It is definitely similar to being a "bad boy!" There are a couple of other meanings that one could read here. It could be a sexual reference related to being a "bad boy." Or, he's singing a song about someone loving him "at his worst," so by saying he'll "do the worst" for her, he could mean that he's going to be himself and try his best in the relationship, and that means showing his "worst." So, strangely, it kind of flips the meaning to "best" within the song's context. Ex: I'll make sure you see that I'm not perfect. => doing one's best to show their worst side Ex: He's the worst thing for me right now => bad boy or not a good match