He also withheld information from the investigators. If you blow the whistle, you're expected to give everything up, not just the stuff that doesn't make you look bad. THAT'S why they nailed him to a cross; he tried to double-deal.
They're fine, thanks. The guy who tries to blow the whistle gets nailed, though.
That's not why they nailed him to the cross, that's how they did it. The why is evidenced by the fact that they're not going after the actual tax cheats in addition to him.