Ad hominem fallacies

Lets try on this sentence I have catch out from a movie called unfaithful.

"You claim that this man is innocent, but you cannot be trusted since you are a criminal as well."

Now let's try working on it:

This argument would generally be accepted as reasonable, as regards personal evidence, on the premise that criminals are likely to lie to protect each other. On the other hand, it is a valid example of ad hominem if the person making the claim is doing so on the basis of evidence independent of their own credibility.

Now give me your comment to see what you got from it.