Okay, so this is just coming from my own perspective. Your perspective/views might be different, and that's totally fine.
The loving kind of harem is all sweet and saccharine. Big happy love fest, everyone gets along for the most part. Nobody is treated as lesser than or whatever. They are all equals and treat each other well. All bright colors with good times and happy vibes. You get the idea yeah? Total fantasy, but they can be really fun to play through, and often have a lot of humor baked into the story lines for an added bonus.
The corruption variety of harem games can usually be split up into 2 parts:
1) Dark Harem: These can get pretty dark. Hell to be completely honest, they are almost always based around abusive and often manipulative relationships and are generally pretty rapey. The MC wil mistreat their "partners" in a variety of ways physically, emotionally, and psychologically throughout the course of the game. Often times it is depicted as a twisted/distorted (or just plain incorrect) interpretation of BDSM with a lot of emphasis placed on Master/Slave dynamics. Generally these games will try to show the MC pissed off at some supposed injustice their target has caused them, and then they use that as a reason to hate-fuck their victim into subjugation, and usually into some form of Stockholm Syndrome. Not always exactly how it plays out, but it's common enough. Another popular variation of this is the use of mind control via hypnosis, magic, or some gizmo to alter the victim over time until they essentially become a sex slave of some form.
2) Fuckfest Harem. These corruption games generally focus on tbe MC trying to find a way to seduce a large number of people in their life, who typically are not romantically attracted to the MC at the start. Might be friends, family, coworkers, other students, or someone you just randomly meet and then chatting with them becomes part of the daily routine. Regardless of the exact scenario, typically none of them want to fuck the MC at the start, and you as the player have to find various ways to get them rhem thinking of you as a sexual being. It can sometimes seem a bit like a loving harem on the surface, but in this one there's typically a much stronger emphasis on conquering and dominating the LIs. You have to keep pushing them and breaking down walls until eventually you convince them that you are in charge of the relationship and they should be happy about it. That they need to need you. Lots of these ones involve slowly exposing the partners increasingly more extreme sex acts until they unlock their hidden inner nympho/slut side. After that they're pretty much always horny and ready for you to shag their brains out any time any where on a moment's notice. Typically much less violent and abusive than the other type of corruption games, but there is still the psychological aspect to it that's pretty manipulative and abusive in its own way.
Again, just my own observations on rhe topic, and I'm not trying to say these are the only ways these games happen.