What's the verb to describe a good guy becoming a bad guy?

One could also use the word turned; e.g.:

He was once an honest politician, but having been exposed to the crime syndicates for so long, now he's been turned.

to corrupt

• to destroy the integrity of; cause to be dishonest, disloyal, etc., especially by bribery.
• to lower morally; pervert
• to alter for the worse; debase.

In your sentence it could be used as:

...but now that he was corrupted, he mingles with...

Other options include:

• debase
• infect
• pervert
• taint
• warp

For a wrestling reference you could use turned heel. In professional wrestling 'good guys' are 'babyfaces' (more commonly just called 'faces'), and the 'bad guys' are heels. When a wrestler changes it's called 'turning'. Thus, a good guy going bad in wrestling is 'turning heel'.