What are some synonymous phrases for the phrase "Turning Criminal"?

Solution 1:

If you're going for subtlety, try

He began to cross the line, committing illegal acts instead of upholding the law...

Other phrases you could substitute here for the bold text would be:

• turn to crime
• cross over
• cross into the dark side (pop culture reference)

Edit: if the character was also homosexual you could make a double entendre with the phrase batting for the other team

Edit2: triple entendre if he was into baseball.

Solution 2:

Become crooked

adjective
4. not straightforward; dishonest.

(dictionary.reference.com)

Almost always meant in a criminal way. (i.e. a crook.)