Stymie was Joe D'Angelo, and he was killed by a Genovese associate

The way I understood, Stymie was killed in a Genovese controlled bar by a drunk Colombo associate who tried to rob the place.
The drunk Colombo man threatened the female manager and this is when Stymie got involved but the Colombo man simply shot him where he sat.

The Colombo's whacked their own guy shortly afterwards because Carmine Persico was being hassled by both the Genovese (because it happened in their bar) and the Gambino's (because Stymie was Gravano's guy)