Everything have read about this story indicates it was suicide.
High-Ranking NYPD Officer Takes Own Life Weeks Before Forced Retirement: Gothamist
Deputy Chief Steven Silks was facing mandatory retirement which seemed to have weighed heavily on his mind of late.
The guy was unmarried, and most of his family lives out of state. That alone can mean some view retirement with dread as they foresee nothing ahead but loneliness
People who have spent their entire lives wrapped up in their careers often become depressed at prospect of retirement or job loss. When getting up and going to work has been all someone has known for > 30 years to suddenly face the fact that won't be happening can be a bad thing. With no wife, kids, grandkids, things just seem that much bleaker. Sometimes even with close family things aren't that better, great film about this is "About Schmidt).
Of course plenty of single retired persons go on to have useful and productive lives. There is help out there to work through these issues, but some people just cannot or will not seek it for various reasons.