The US arms industry effectively buys the votes of senators deciding whether to approve an arms sale to a repressive theocratic regime such as Salafi Arabia.

Should we condemn individual senators for selling their votes, or the system that pressures them to do this?

Both! Of course, both!

We can forgive people who are in positions of weakness for yielding to the pressures of the system, except when they do something egregious. However, someone as powerful as a senator can never plead, "the system made me do it."