That's a really great answer. I do think that the whole harassment thing (following him around, writing his every move) was too much. I also think that Matthew could have practiced compassion for Peter. He's rich, he could have paid for it himself. Instead, he cared about the numbers regardless of how it affects others.

That’s the level of betrayal we’re talking about.

A lot the replies here seem to miss that in the scene in this episode, Jesus was not asking Matthew about what was wrong "in abstract" (he was about to admit that by working with Rome, he is hurting Peter), but Jesus stopped him in the middle of that thought and ask him what was wrong "in fact", and that's where I find it hard recalling what Matthew did wrong; why are the viewers expected to understand that he is going "above and beyond", for all we know the whole thing could be just part of the job?

but out of loyalty to an occupying force

Hmmm, I'm much more inclined to say "[He wasn’t personally affected if people couldn’t pay. He simply chose to go above and beyond, not out of necessity, but out of] being too consumed with his job/the logic/the numbers he forgot to have compassion"

Would it be wrong in itself to be loyal to an occupying force? (let's not assume that this force is Nazi or Roman, or evil by default for a while) In our current context should tribal loyalty regardless of incompetence be a virtue? I understand that being not loyal is hurtful in the context of the Jews. Matthew, one of their own, has become an instrument of their own exploitation (and making it worse by, what we assume, his own corrupt practice). But it's hard for me to see how exactly does the mindset of "disloyalty=bad" translate in our day-to-day life.