Suspect: ‘I didn’t kill nobody’
It's a stereotype of people of a lower class, minimal education or typical thugs to use non-grammatical English. And one of the common examples is the incorrect use of a double negation like this. Don't say this in real life.
This is a matter of dialect. English has many dialects, and they are not simply regional - they are heavily dependent on class culture as well. Dialects considered "less educated" often feature this sort of emphatic double negative, the two negatives resolving to a stronger negative.
This was actually common among more dialects up to the 18th century, when prescriptive grammarians started wanting to describe English in a logical way. As English did not (and does not) function terribly logically, this required coming up with new rules. They liked to find ways to make the rules that applied to Latin fit English, so they decided that double negation led to a positive in much the same way. After that, "properly educated" English speakers were expected to treat double negation as it is treated in Latin.