In announcing that he intended to vote to find Donald Trump guilty of abuse of power, Mitt Romney absolutely did not mince words, saying "Corrupting an election to keep oneself in office is perhaps the most abusive and destructive violation of one's oath of office that I can imagine."
His announcement focused on that one charge. On the other article of impeachment, for obstruction of Congress, Mitt voted to acquit.
In doing so, he exposed the hollowness of his high-minded words about his duty and the public trust. True, as he pointed out, his guilty vote was not the deciding factor on that charge, and nor did his vote to acquit make a difference on the second one. But because he voted differently on each one, we can infer that there is a meaning to his votes.
In voting for the charge of abuse of power and against the charge of obstruction, Mitt essentially joins his colleagues like Susan Collins and Lamar Alexander in saying that he believes that what Donald did was wrong but he doesn't believe Congress should have the power to hold him accountable. If he was trying (as they obviously did) to hedge by simultaneously condemning and protecting The Tan Who Would Be King, he'll probably be disappointed. Trumpland was already willing to portray him as a bitter NeverTrumper RINO who is angry and humiliated at being passed over for Secretary of State.
More likely, I think his split decision reveals something sincere: that he does deplore Trump's use of power but he would like to think that the president is above the petty purview of Congress. Not that Trump specifically is, but that presidents — Republican presidents, at least — in general need to have the power to blow off subpoenas and ignore court orders and make vague, extralegal declarations of nebulous forms of privilege in order to keep doing as they will do.
It's the same reason that Trump's poorest, least enfranchised fans cheer on his lavish lifestyle, defend his tax cuts for the rich, and applaud his attacks on the social safety net: Mitt can see himself where Donald is, some bright blessed day, and when that day comes he doesn't want the record to show that he believes a president can be called onto the carpet by Congress.
In short, Mitt Romney was not voting as a senator, but as a temporarily embarrassed president.