I assume reaper got tortured a lot. Broke his mind and in his broken state he latched onto something to stay sane-ish. He assumed his friend had abandoned him. Maybe he even knows deep down it didn't happen that way but he had to hold on to something to get him through hell, and hate and anger are easier to grab onto...especially for warriors...than hope and faith.
As for the Ex...I assume at this point she's just a flat out sociopath. That word is used as if it's synonymous with psychopath, but it's not. A sociopath...in simple terms...is a person who is incapable of empathizing with others. They don't even see other people as 'people'. To a sociopath everyone around them....even their own children...has about as much value as an NPC in a videogame. You may get more attached to some NPCs than others but at the end of the night you can still detlete them all without losing sleep over it.
To a sociopath your worth is determined by how you make them feel. If you please them, and they enjoy your company they can be very loyal and devoted and passionate....right up until you no longer amuse them. Or until someone new makes them feel even better. The only restraint on a sociopath's behavior is what calculation they reach when they perform a risk-reward analysis. High functioning sociopaths are the ones who are better at anticipating how others will react to their behavior and factoring that into their risk-reward analysis. That also makes them the most manipulative and cunning.
That lack of empathy certainly explains why she cheated....the same way we as H-Gamers often take cheating paths with every available NPC even if we genuinely like the romance path with one particular NPC. She didn't think she would get caught, so why not enjoy a thrill with some other guy and then go back to her "Romance Storyline" with the MC as if nothing happened?
That same lack of empathy explains why she would work for the kind of people who create zombie experiments. She's the kind of person they NEED. No moral conflicts. It also explains why she would not hesitate to sacrifice her own cousin if she thought it would get her the MC back. Sociopaths HATE losing. They can and will go to extreme lengths to win back a lover they lost...and then go right back to screwing around on them if they believe they can do so without getting caught again. Or if they enjoy the dance of losing their lover and then winning them back again and again. It just depends on what kind of thrill, excitement, or satisfaction the sociopath is getting from the experience. Because at the end of the day even the lover is just an NPC. Just because that lover is their favorite NPC doesn't mean they're not still an NPC.
If Diane places greater value in how the MC makes her feel than on how the cousin makes her feel well...sorry, not sorry Zoey. So I imagine Diane got recruited to work for the evil overlords at least in part because of her amorality, and perhaps she went off reservation a little bit when she got to figuring that she could kill two birds. Get her favorite NPC back, and give her bosses a bunch of healthy young test subjects.
