people go to prison for murder every day
what Frank gave the cops was once in a lifetime
the sort of thing immunity can't buy

He didn't kill any innocents, that is for sure.

As far as I remember, the only person he was seen killing was his rival at the start. Presumably nobody reported it and the cops probably didn't care enough to follow it up, especially since he was a nobody at that time.
Other than that he kept his own hands clean so he would have been hard to convict for murder. Instead they got him on other charges that would have still normally added to 70 years in jail.

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they got what they wanted- convictions for crooked cops

They didn't have any evidence of him murdering anybody. Same thing with a guy like Al Capone, everybody knows he murdered or had people murdered but he was only convicted of tax evasion.
"And it made me wonder, would I do the same? What would I sacrifice to be loved? Not my balls."

Well, he did get fifteen years. It's not exactly the harshest punishment but it's more than nothing.
The knack to flying lies in learning how to throw yourself at the ground and miss.