And I think what many of us in this thread object to is that your specific question is irrelevant, because it's hypothetical and unrealistic. You
do not know the money is for a hit job. You just don't. You may
think that's what it's for, but you don't actually
know, and you -- as a random employee (or worse, algorithm) of Payments R Us, Inc. -- should have no business playing judge and jury.
As for moral lines you are curious as to what people will and won't cross, those lines will be different for different people, and that's exactly why we shouldn't be putting these sorts of decisions in the hands of random unaccountable corporate employees (or, again, worse, algorithms).
You clearly want someone to say "no, of course I wouldn't allow a transaction to go through if it was for a hitman contract". But what if someone said "I see no problem with contract killings; I'd let it through"? That person could be working at Payments R Us, and clearly we don't want someone making that decision! So take it out of their hands.
The contract killer is of course an extreme example, but we already have real-world examples discussed here that illustrate the problem with this. Pornography is legal, and yet payment processors boot online porn companies from their networks. Someone who thinks pornography is immoral might have made that determination (I don't buy the "high risk of fraud" argument; you can always control for that with higher fees). Regardless of how you personally feel about pornography, do you think it's ok for a company to deny another company access to a big chunk of the financial system for entirely legal activities, just because they don't like them? I would sincerely hope that we can agree that sort of thing is bad.