The Hellcats (1968)

A soldier comes back from the war and goes undercover to avenge the death of his cop brother. His link to the murder is the fiancée of his brother, who wants vengeance too, and knows he was an infiltrated cop with a drug-dealing biker gang, the Hellcats, who are employed by the mob to move the drugs from Mexico to the USA. The two would-be avengers infiltrate the Hellcats, the girl is abducted, and now it's the retired soldier against all the bad guys.