Once a gangster, always a gangster.

Joseph “Joe the German” Watts, an aging top henchman of the late John Gotti, yesterday was hit with a maximum prison term by a judge who noted that the old Mafioso had resumed his murderous life of crime “almost immediately” after getting sprung from the slammer the last time.

“He does not appear to know how to live his life without violence and killing,” said Manhattan federal court Judge Colleen McMahon of the 69-year-old Watts, slapping him with 13 years for the murder of a suspected snitch in 1989 and the beatdown of a former prison pal who failed to make him money in the stock market.

Watts — the reputed backup shooter at the famed 1985 rubout of Gambino boss Paul Castellano — had walloped the fellow ex-con a few years ago, after just completing a six-year sentence for money laundering.