To flip that around sort of, you could have the villain be a known antagonist inside our own faction.
A general or a politician who is part of our system and can't just be killed and we have to work alongside effectively, but have conflicting goals with. Sort of Udina + Kurtz + Palpatine. Or Saren if we hadn't proved he was evil as part of our protagonist club initiation ceremony.
I was thinking Udina, actually, but figured that'd be among the less desirable Bioware characters to bring up, lol.
What's interest about a rival-faction-ally is that there's the opportunity to try for conversion. You try to convince them, and they try to convince you. The fact that they try to change your mind, rather than dismiss you, serves as a sort of proof of interest, and the idea of 'winning them over' serves as an alternative metaphor for the chase/struggle/conquest/catch.
While, personally, I think a good 'evil' love interest should be self-confident enough in themselves and their beliefs to convert for you (the player should, after all, be interested in who they are, not just what they wish they would be), the idea of love/interest being enough to... I wouldn't say 'love redeems,' but maybe 'consider compromise' could be an interesting idea. If you could avoid the idea that they convert to your point of view- possibly by necessitating the player compromise to theirs- you could use a minor change as a significant character evolution.
To pick a Mass Effect context- imagine the love interest is a Ruthless Pragmatic Renegade. They have emotions, but are far from bleeding hearts. Not the sort of compromise or make concessions to terrorists even if they take hostages. A key point in the romance could be whether you accept that or not. Paragon is too good to accept losses, and so takes great risk and risks a potential break-up fight to attempt a hostage rescue. Renegade accepts his/her logic, and the losses, and shows understanding of the viewpoint they bring to the table.
But, later on it's the player who's in dire straights, and it's the Renegade LI who has to make the choice- possibly a deliberate setup by the Ruthless LI's enemies to hurt him/her. Be ruthless, and sacrifice you for the safety of the greater good? Or do what he said he wouldn't, and try for a better ending?
In such a setup, the Ruthless LI backing away from the 'Renegade' option- and trying to save you instead- would be a pretty cool and significant step for them. Character evolution in that they're not as heartless as they seemed (or tried to be). Emotional significance for the player, since the PC caused them to take that step. And then allowing fallout and emotional release afterwards, as the LI and PC reunite and accept the consequences and fallout.