In Amazing Spider-Man vol. 2 #51, a green-skinned menace named Digger starts stalking a bunch of New York's old mob hideouts—and murdering the gangsters he finds inside. That puts a local don, Mr. Forelli, into a panic. Thankfully, he has a solution: hire Spider-Man to work as his bodyguard.
Spider-Man turns Forelli down, citing moral reasons—because, y'know, Forelli is behind the bulk of the city's sex trafficking and drug dealing operations—until Forelli offers to pay Spider-Man $10,000 a day (plus expenses), at which point Peter Parker folds immediately and starts working for the mob.
Now, technically, Forelli doesn't ask Spider-Man to do anything against the law, but defending Forelli so he can keep running his illegal smuggling enterprise is suspect enough, as Mary Jane rightly observes. Still, Peter Parker quickly settles into the jet-setting life of a professional criminal. Along the way, he discovers that Digger is actually a composite of 13 gangsters Forelli killed back in the '50s, whose bodies were buried in a chemical dump before a gamma bomb fused them together and brought them all back to life.
Spider-Man promises to stop Digger, but raises the rate to $12,000 a day. Before long, Digger attacks, but the gamma radiation that melded all the corpses together begins to wear out, and Spider-Man battles him until he literally falls into pieces. After the job is finished, Spider-Man hands over a tape full of evidence to the NYPD. Forelli is arrested, and Parker keeps the money.
So, to summarize: Spidey defended a murderer, stole his money, effectively killed over a dozen people (again), ratted out his boss, and got away scot-free. Honestly, if the whole crimefighting thing doesn't work out, Parker has a pretty bright future ahead of him as a mafioso, as long as he can keep himself from screwing it up (which, let's face it, he probably will).