Lone Wolf #11

Burt Wulff, after two years in the army, most of it in Vietnam, had been entitled to something nice for his trouble, so they had made him a narco. A New York City narcotics cop, with the freedom and the plainclothes and the graft money... but something had happened to this Wulff overseas: he had gone crazy. He had become a man of integrity. Eventually he tried to bust an informant, and they knew they would have to do something about this wild man...
When Wulff saw his fiancée OD'd out on the floor, he thought that he might go mad on the spot but quite strangely he did not. Wulff went straight home and discarded everything except his gun and a spare. They were hardly the equipment he would need but they were a beginning. By mid-summer, he had the beginnings of an operation in his mind. The rest he would have to play by ear. Wulff hit the streets to kill a lot of people.

The 11th and 12th books in Malzberg's vigilante series, The Lone Wolf, originally published by Berkley Books in the early 1970s under the name "Mike Barry."