These sound good: "Nobody will miss him if he doesn't come back." "Nobody would miss him if he didn't come back." "Nobody will be missing him if he doesn't come back" "Nobody would be missing him if he didn't come back" Slang, horrible, very bad English, but sometimes spoken: Ain't nobody gonna miss him if he don't come back.

It's hard to really tell without more context. It could be correct as is, depending on the situation. For example: Imagine the bad person went on a dangerous mission. He didn't come back. Maybe everyone else doesn't know it yet. , So the act of him not coming back is in the past, but the act of nobody missing hasn't happened yet, i.e. is still in the future. Also, you have to remember that characters don't always speak gramatically correctly.

My guess is that the first part is “Nobody was going to miss him ….” I would be rare for a native speaker to misspeak ‘didn’t’ for ‘doesn’t’.

Yes, doesn’t is correct if future tense is used in the first half of the sentence. If you say “nobody would miss him” then you could say “if he didn’t come back”.