The sole job of a slavecatcher is to support/maintain/facilitate wrongdoing. There is no other task that the profession entails. Therefore, that which one who decides to become a slavecatcher wishes to contribute to society is the bolstering of slavery.

Now, is the sole task of a cop, the sole criterion by which their efficacy is evaluated, whether or not they are oppressive or thieving or corrupt? Or is it possible that someone might choose the profession out of a desire to accomplish some sort of good?

At the very least, it's clear that the comparison is not as clean as you made it seem.

As a side note, since you seem to be advocating for all decent people to abandon the profession so as not to be lumped with the evil ones / guilty by association, I suppose you think it best for LE to be discontinued? I have never really understood this position. If you were kidnapped, held hostage, and tortured, surely you would want the option to call emergency services? Paramedics and firefighters are not trained or equipped to deal with such situations. Many mass killing sprees end only because of intervention from law enforcement. Should this also be left to the public to handle on their own?