Police have arrested Shakeem Parker for the killing of notorious drug-dealer-turned-snitch Alpo Martinez.

The 27-year-old Harlem resident was already in custody on Rikers Island for a previous gun case.

There was an assumption that Alpo's murder was drug-related.  The 55-year old had ditched his witness-protected life in Maine and the word was he was back in the NYC drug trade.

However, the cops say that has nothing to do with why Parker shot Alpo to death while Alpo sat in his car on a Harlem street Halloween night.

They say that Parker was angry with Alpo because Martinez, who has a history of reckless driving, had hit Parker while he was on his motorcycle months earlier.

Parker happened to notice Alpo's SUV on Halloween and used the opportunity to get his revenge.

 “There’s no indication that this was anything other than a crime of opportunity," said a police source.

The shooting was caught on camera.