He should not resign. He should stay in office and continue to vote the Republican agenda. He owes that much to the voters that elected him, and the people that actively supported his campaign.
Is the indictment legitimate? Everyone believes the worst of pols when they're indicted, but Harold Ford, Sr. (D-TN) was under indictment for years. There is no reason for Santos to resign, and the moral outrage over him is generated entirely by Democrats looking to take the House back by attrition, while stopping the investigations into Biden. Santos's vote in Congress is what matters. His vote has the same impact as the vote of a paragon of virtue and is more constructive, from my point of view, than the vote of a personally virtuous Democrat who will support policies I generally oppose.
So you’re saying Republicans should treat voting the party line as their thirty pieces of silver?
I'm saying that Santos is a duly elected member of Congress. He will vote for policies I generally support. His Democratic replacement will not. If the House wishes to expel a member they can try, but I would not support it unless he were convicted of a felony.