Three words explain the downfall of ex-Brooklyn District Attorney Joe Hynes, who allegedly used staffers and public money for his sixth re-election campaign last year.

Those three words are ego, ambition and cynicism. Hynes aspired to be mayor and ran unsuccessfully for both attorney general and governor.

Now, a NYC Department of Investigation probe has implicated him in the improper use of both senior staff during his 2013 campaign as well as money seized from criminal defendants to pay a political consultant. The state attorney general reportedly subpoenaed Hynes' aides Monday.

Hynes' tenure underscores the need for term limits for city district attorneys. Their terms are four years, but once elected, they have lifetime jobs:

In Brooklyn, Hynes was first elected D.A. in 1990. He is closing in on 80.

In Queens, Richard Brown has been D.A. since 1991. At 82, he plans to run again next year.

In the Bronx, Robert Johnson has been D.A. since 1989.

The granddaddy of them all, Robert Morgenthau, served as Manhattan D.A. for 34 years. He retired in 2009 as he approached 90.

Only if a district attorney does something truly outrageous can he be pressured to quit.

Brown was appointed Queens D.A. after his predecessor, John Santucci, quit when he could not explain his 14-hour lunch with Sal Reale, a reputed associate of the Gambino crime family.

Hynes' reputation grew when, as a special prosecutor, he took over the racially charged Howard Beach case and successfully prosecuted a group of white youths who chased Michael Griffith, 23, who was black, to his death on the Belt Parkway in 1986.

In 1989, he considered running for mayor but said he did not want to run against David Dinkins. Instead, Hynes ran for Brooklyn D.A.

Because he lived in Breezy Point, a cooperative that included few Jews, his opponent accused him of anti-Semitism. Hynes may be many things, but an anti-Semite isn't one of them.

It became apparent he felt Brooklyn was a small venue. In 1994, a year after his first re-election, he ran unsuccessfully for attorney general. In 1998, he ran unsuccessfully for governor.

It was around this time that he courted the Hasidic community and seemed to lose his bearings.

After Rabbi Abraham Rubin of Borough Park was kidnapped and beaten in 1996, Hynes aide Michael Vecchione promised swift justice. No one was prosecuted.

In 1995 Vecchione successfully prosecuted Jabbar Collins of Brooklyn for the murder of Rabbi Abraham Pollack. Collins spent 16 years in prison before revelations of possible prosecutorial misconduct led to his release in 2011. In overturning the verdict, federal Judge Dora Irizarry ruled that Hynes' office "had wrongfully withheld a key witness' recantation, had knowingly coerced and relied on false testimony and argument at trial."

Collins' $150-million lawsuit in 2013 named nine assistant D.A.s, detective investigators and Vecchione as defendants. Federal Judge Frederick Block, who presided over Collins' civil trial, said he was "disturbed" and "puzzled" that Hynes continued to praise Vecchione. "This was horrific behavior on the part of Vecchione," Block said. "We are going to have a civil proceeding and all of this is going to be uncovered." The case continues.

Last year, David Ranta, another man apparently wrongly convicted, was released after doing 23 years for the 1990 murder of Rabbi Chaskel Werzberger. The New York Times reported that Hynes' office sent Ranta and others to prison on false evidence turned up by former Det. Louis Scarcella, whose work is under scrutiny.

One last point: The head of the Department of Investigation, Mark G. Peters, ran for D.A. against Hynes in 2005. Peters also donated $500 to the campaign of Kenneth Thompson, who beat Hynes last year. Why Peters did not recuse himself from the inquiry remains unclear.

CORRECTION: Eugene Gold was no longer Brooklyn district attorney when he admitted fondling a child in Tennessee in 1983. His official status was described incorrectly in an earlier version of the NYPD Confidential column.