Love is not selfless....well, maybe that romanticized "true love" is, but the "love" that most people experience is anything but selfless.
To quote Bender: "Bender knows love, and love doesn't share itself with the world. Love is suspicious, love is needy. Love is fearful, love is greedy. My friends, there is no great love without great jealousy!"
Also: as for turning her into some sort of monster, the trope here is pretty standard and provides a few options:
A: She is monster-ized only physically. Because of this strange new body, the powers of darkness flowing through her, she finds it difficult to master her emotions and control herself. She's not evil, she's just scared, frustrated, angry and sad.
B: she is monster-ized mentally(and physically). This can range anywhere from complete brainwashing through overwhelming her mind with evil thoughts, seduction towards evil Darth Vader style, torture and blackmail(I will kill your babies and loved ones!) forcing her to do evil against her will, and so on.
-Depending on how you run it, this can either be very good or very lame. Choice A almost always comes across as a little cheesey, but it can be very endearing when a character tries their hardest to to "mind over matter" fight their new evil-inclined body.
Choice B has it's up's and downs as well. Slow falls into evil are often great, but they are delicate, she needs motivation and a certain willingness(which must be more than "I'm supernaturally inclined towards evil!) to do destructive, evil acts. Often these are done for the "greater good", but such things are generally considered to be a slippery slope. As Yoda says: "Once you start down the dark path, forever will it dominate your destiny." Evil acts in the name of a greater good are seductive because they are usually easier and more obvious, but when you do them long enough you lose sight of what is actually right and wrong.
Complete brainwashing into evil is less interesting, but fun if you like a nearly comedic level of moustache-twirling villiany. I say it's "less interesting" because it's usually too easy. The villain has evil magic powers that made you evil, so now you're evil, until ya know, you get the spell reversed and you're not. There's no character depth to these sorts of plots, which is why they're best played up in a fairly comedic manner rather than a serious manner.
-I should add, the reason complete brainwashing is usually uninteresting is because it requires a lot of setup in order to properly execute. If the answer to why your character did an evil act is "well because they're evil", then it's meaningless and treads into the over-the-top comedy villainy. Evil acts now due to brainwashing work best when they're built upon past character development. Lets say for example at some point your character couldn't decide between two men on who to date/love/marry. Now that she's evil, she makes the one she chose feel horrible and betrayed by sleeping with the one she didn't. Or maybe perhaps your character saved the child or friend of an ally, so now that she's evil, she goes out and tortures/kills that person. Brainwashing into evil is great in these sorts of instances, betrayal is a feeling that should stick with any character even after your character "becomes good again"(if ever). So that even when you are good, you are distrusted and shunned for your evil actions.
To quote Stalin: "One death is a tragedy, a million deaths is a statistic." And in many stories and settings, this is true. It's easier to forgive "great evil", such as burning down a church or slaughtering a small down on the basis that you were brainwashed into evil. However it's much harder to get over personal hurts, as mentioned above. "Oh well you were evil." Doesn't truly assuage the feeling that your now-good-again friend murdered your daughter.
Blackmail asks a bigger question of what rating you would give your game, G? PG? PG-13? R? NC-17? Blackmail gets into all sorts of nastiness and it is NOT a route for the faint-of-heart DM. If the truly evil greater demon kidnaps the holy knight's child and attempts to blackmail the paladin into doing evil acts, the DM MUST be willing to go through with the devil's threat if the paladin says "NO." If the DM is unwilling to play such depravity, then what should have been a powerful, feared enemy becomes a joke. He instead throws the baby aside and disappears in a poof of smoke, his plans foiled because some holier-than-thou guy said "No." What a wimp. Blackmail can entail all sorts of horrible things that I DO NOT recommend be played out with any party whose members can't, on their own, watch an R-rated movie. That said it can create very interesting games with deep stories and very evocative moments.
As for the exact math of how one becomes a devil over a demon is one of those things that can be debated for ages. In a D&D game, I take the shortcut and say "He's evil, so he's got super-evil magic that can make non-devils into devils...so....*POOF* you're a devil!" Otherwise, technically speaking devilry is more of an alliance to evil, while demons are the more naturally-occurring monsters of the shadows. So "becoming a devil" is less an issue of genetics and more one of alliances. You serve the devil-god? You're a devil. Even if technically you're a demon, or a treant, or a potato.