So you've swallowed the pill. You're a reforming nice guy and you think,
"I can still be nice. Knowing the truth doesn't change who I am."
"Sexual strategy may be amoral, but I'm not."
"I don't want the reputation of being an asshole."
Well, you can be nice if you want to, but remember that the only benefit to being nice is that you will think that you are nice. To her, no matter how Nice you are, you WILL be an asshole and it will be your fault that you broke up.
My father gave me one bit of advice after my breakup.
"Don't tell anyone anything. If they ask, just say 'It didn't work out. We split up.' Don't go spreading around a bunch of hurt and drama and details."
Originally I thought it was because he was trying to give me moral advice, but in reality it was because men don't do that, women do.
We know women see the world through their feelings. We also know that women will perform whatever mental gymnastics it takes to make those feels be good.
Scenario: Your LTR cheats. You find out and next her. You don't yell or otherwise abuse her, just "goodbye". Good job for standing up for yourself while being kind. You handled it perfectly. However...
She's not going to be able to sleep at night being the bad guy. She will hamster it to make herself feel like the victim.
"How could he just walk away after all we had?"
"I thought he loved me. Didn't I mean anything to him?"
"It was just a mistake! How could he be so cruel?"
"It's not my fault, he was neglecting me!"
Squeak, squeak, squeak goes the wheel, until her hamster has nearly convinced her that it's not her fault. She only needs one more thing: approval.
She'll go to her friend, or she'll go to Chad, or she'll go to her mom, and she'll spill her heart out. She'll cry and paint a picture of you: cruel, neglectful, an asshole.
"There, there," they'll console her. "How could he do that to you? He's an asshole. You don't deserve the way he treated you. It's not your fault."
"Ah, that feels better." Says the hamster-brain. "I knew it wasn't my fault I cheated."
How did this happen? You were perfectly nice, but now, your mutual friends are avoiding you. Chad says he'll kick your ass if he sees you. Any blue pill thoughts you had about a possible post-breakup-friendship quickly evaporate. Congratulations, being nice got you nowhere.
What do you do?
The choice is yours. You can live by whatever chivalry or code or lack thereof that you decide, but remember that it's not what she'll say about you.
In the end, just like everything you do in TRP: It's not about her, it's about you. If you decide to be honorable, it's only for your own conscience.