[to **corrupt**][1]

> * 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...

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Other options include:

* debase
* infect
* pervert
* taint
* warp

[1]: http://www.dictionary.com/browse/corrupt