In law enforcement, as in most every other profession, there are good ones and bad ones, but what most people seem to remember are the really bad ones.

Manuel Pardo, a former Florida police officer turned serial killer who was executed Tuesday, was one of the latter, officials say. He shot nine people to death in the late 1980s, claiming he was a ”soldier” ridding the streets of the wicked.

Most of Pardo's victims were reportedly involved with drugs, and Pardo claimed he was doing society a favor by ridding the streets of low-lifes who “have no right to live.” Authorities say he was a cold-blooded serial killer; one retired detective described him as “Ted Bundy-esque," while a retired prosecutor called him “very cold.”

Michael Tabman, a former Fairfax County, Va., police officer and former FBI agent, said law enforcement agencies have better screening tools these days to weed out potential problem applicants, "but we haven't perfected predicting behavior."

'Death Row Romeo' faces execution in Florida

He said people attracted to police work often have personalities that are "machismo-oriented" and "comfortable with a lot of authority," among other traits.

"A lot of that is a type of personality that in a perfect storm … can morph into anti-social behavior," said Tabman, an author who also blogs about crime and security. 

Herewith are some other notorious cases involving cops gone bad:

M. Spencer Green / AP file

Former Bolingbrook, Ill., police Sgt. Drew Peterson arrives at the Will County Courthouse in Joliet, Ill., for his arraignment on charges of first-degree murder on May 8, 2009.

Drew Peterson
The former Bolingbrook, Ill., police sergeant was convicted in September of murdering his third wife, Kathleen Savio, who was found dead in her bathtub in 2004. Authorities presume his fourth wife, Stacy Peterson, who vanished in 2007, is also dead; Peterson is a suspect but has never been charged in that case.

Before his 2009 arrest, according to media reports, Peterson seemed to taunt authorities, joking on talk shows and even suggesting a "Win a Date With Drew" contest.

After his conviction, Savio’s family members said justice was finally served. "Game over, Drew," Stacy Peterson's sister, Cassandra Cales, said. "He can wipe the smirk off his face. It's time to pay." 

Via KTLA / AP file

A frame from a video shot by George Holliday from his apartment in a suburb of Los Angeles shows a group of police officers beating Rodney King as other officers watch on March 31, 1991.

Rodney King beating
It was perhaps the most famous of all homemade videos – the 1991 clip of Los Angeles police officers beating black motorist Rodney King following a car chase.

A year later, a California jury acquitted three officers and deadlocked on charges for a fourth. The verdict sparked violent race riots in Los Angeles, and by the time order was restored, more than 50 people had died.

A federal jury later convicted two of the police officers, Stacey Koon and Laurence Powell, of a federal charge of violating King’s civil rights and sentenced them to 30 months in prison.

King died in June at age 47.

Katrina bridge shootings
In the chaos after Hurricane Katrina, six unarmed civilians were shot – two of them fatally – on the Danziger Bridge in New Orleans on Sept. 4, 2005. One of the dead, Ronald Madison, was a 40-year-old mentally disabled man who was shot in the back. Police claimed they opened fire because they thought people were shooting at them from the base of the bridge.

Getty Images file

Cars pass over the Danziger Bridge July 14, 2010 in New Orleans.

In August 2011, four former New Orleans police officers -- Kenneth Bowen, Robert Gisevius, Anthony Villavaso and Robert Faulcon – were convicted of civil-rights violations and firearms and other charges in the shootings. A fifth former officer, Arthur "Archie" Kaufman, who was assigned to investigate the shootings, was convicted of helping to orchestrate a cover-up.

“The officers who shot innocent people on the bridge and then went to great lengths to cover up their own crimes have finally been held accountable for their actions,” Thomas E. Perez, assistant attorney general for the Justice Department’s civil rights division, said when the men were sentenced to long prison terms in April.

Reuters file

Former New York City police officer Justin Volpe in 1997.

Abner Louima beating
Louima, a Haitian immigrant, was brutally beaten and sodomized with the handle of a toilet plunger in the bathroom of the 70th Precinct station house in Brooklyn, N.Y., after being arrested outside a night club in August 1997.

One cop, Justin Volpe, was sentenced in 1999 to 30 years in prison for what the judge called an “unusually heinous” crime and a “barbarous misuse of power.” Another, Charles Schwartz, who was initially accused of holding Louima down, pleaded guilty to perjury and was given a five-year sentence. Two other officers who were indicted for allegedly trying to cover up the assault had their convictions reversed due to insufficient evidence.

Chicago Sun-Times / AP

Former Chicago police officer Jon Burge, convicted of lying about the torture of suspects, walks to his attorneys' office following the first day of his sentencing hearing at the federal building in Chicago on Jan. 20, 2011.

Louima sued New York City and its main police union and won a $8.75 million settlement.

Watch US News crime videos on NBCNews.com
 
Jon Burge
Burge was a former Chicago Police Department detective and commander who, along with the “Midnight Crew” of officers under his command, allegedly beat and tortured criminal suspects in the 1970s and '80s in order to gain confessions. Victims said they were burned with cigarette butts, smothered with plastic bags, shocked in the genitals and forced to play Russian roulette with a .44-caliber gun.

Although Burge was protected by the statute of limitations on the claims of abuse, he was convicted of lying about the torture. He was sentenced in January 2011 to 4 ½ years in prison.

The city agreed to pay more than $7 million to settle two torture lawsuits involving Burge.

More content from NBCNews.com:

• Amid protests, Michigan passes anti-union 'right to work' measures
• Gay student asks Justice Scalia to defend his 'bestiality' comments
• Marijuana sales? Lots of obstacles still to weed out in Washington, Colorado
• California exodus as thousands quit state for Texas, Ariz., elsewhere
• Banking giant HSBC to pay record $1.9 billion in money-laundering case
• Video: Massive gas line blaze caught on camera

Follow US news from NBCNews.com on Twitter and Facebook