Admittedly I'm not 100% sure about this one, but I figured it couldn't hurt to ask! Not counting the Reo & Mabu manga or Twitter since he's the protagonist of those, and not counting the manga by Migii since there's not much of it. I allude to the light novels but he's not quite as villainous there due to narrative framing so wasn't sure if I should put that in the title of the comment.
Pros: Reo does a lot of reprehensible stuff in canon and is firmly positioned as an antagonist for the first half of the anime. He's a cop that serial murders civilians, almost always for pretty bogus reasons, and the timeline of Sarazanmai implies that he does this for over a decade. These civilians turn into Kappa Zombies, which then cause problems for (at the very least) all of Asakusa, up to and including mind controlling women to be sacrificed. He is also shown to mind control people who witness him committing crimes, which allows him to get away with it. The fate of the people he kills is worse than death - after the protagonists fight the Kappa Zombies, the Zombies are entirely erased from existence, with nobody able to remember them at all. Reo shows no restraint when it comes to who he is willing to target for murder - he kidnaps a prepubescent child with the intent to murder him, and also shoots a teenager to further his own needs, and proceeds to laugh in the teenager's face more than once. ("Oh, you're the kid I shot? Sucks to be you!") The narrative does not show these actions as good or justified, and his love interest/partner in crime calls him out on using children as going overboard. He also emotionally and verbally abuses his resurrected love interest, which is also shown as not justified. He is so focused on his own goal that he doesn't see anything else, and is willing to stoop to any level to make sure it becomes a reality. A smaller thing: he kicks children and animals.
Cons: While I would argue that the show didn't agree with what he was doing (word of God more or less said that he was being stupid and thoughtless), and that he went above and beyond in terms of cruelty, he was more or less a prisoner of war to Otter (the Big Bad) and was emotionally compromised due to the death of his love interest. Also, the "erasing people from existence" thing isn't necessarily his fault and more the rules of the universe, even if he could have prevented it by not killing people. The light novels are more sympathetic to him than the anime, as they show flashback scenes that show his emotional point of view a little bit better and explore backstory that make it more obvious that his love interest is the only person who has consistently humanized him, so he's kind of a hot mess without him - and his goal in the anime is to bring his love interest back to his pre-death state. I could go into the details of that if you need more info but their whole relationship is pretty complex. Another con is that there is a brief text scroll that says something like "I don't know why Reo and Mabu are doing the things they're doing, but I can't believe they're actually bad people" around the halfway point in the anime, which comes from the point of view of the "Greek Chorus" character (although this character is also their in-canon daughter, so keep that into consideration too).
In terms of fandom, he's an extremely polarizing character - people usually either love him and think he did nothing wrong since it was all for love, or people hate him. I think it's hard to not be sympathetic to him by the end of the anime due to a combination of an emotional redemption arc and being shown how much he went through to become the person he was in the anime, but at the same time... the show was pretty adamant on saying "Wow this asshole tried to kill kids, and abusing your authority to kill people/isolate people isn't great" in my opinion. That said, this show is pretty abstract/symbolic so YMMV.
Sarazanmai (anime, 2019) - Niiboshi Reo
Pros: Reo does a lot of reprehensible stuff in canon and is firmly positioned as an antagonist for the first half of the anime. He's a cop that serial murders civilians, almost always for pretty bogus reasons, and the timeline of Sarazanmai implies that he does this for over a decade. These civilians turn into Kappa Zombies, which then cause problems for (at the very least) all of Asakusa, up to and including mind controlling women to be sacrificed. He is also shown to mind control people who witness him committing crimes, which allows him to get away with it. The fate of the people he kills is worse than death - after the protagonists fight the Kappa Zombies, the Zombies are entirely erased from existence, with nobody able to remember them at all. Reo shows no restraint when it comes to who he is willing to target for murder - he kidnaps a prepubescent child with the intent to murder him, and also shoots a teenager to further his own needs, and proceeds to laugh in the teenager's face more than once. ("Oh, you're the kid I shot? Sucks to be you!") The narrative does not show these actions as good or justified, and his love interest/partner in crime calls him out on using children as going overboard. He also emotionally and verbally abuses his resurrected love interest, which is also shown as not justified. He is so focused on his own goal that he doesn't see anything else, and is willing to stoop to any level to make sure it becomes a reality. A smaller thing: he kicks children and animals.
Cons: While I would argue that the show didn't agree with what he was doing (word of God more or less said that he was being stupid and thoughtless), and that he went above and beyond in terms of cruelty, he was more or less a prisoner of war to Otter (the Big Bad) and was emotionally compromised due to the death of his love interest. Also, the "erasing people from existence" thing isn't necessarily his fault and more the rules of the universe, even if he could have prevented it by not killing people. The light novels are more sympathetic to him than the anime, as they show flashback scenes that show his emotional point of view a little bit better and explore backstory that make it more obvious that his love interest is the only person who has consistently humanized him, so he's kind of a hot mess without him - and his goal in the anime is to bring his love interest back to his pre-death state. I could go into the details of that if you need more info but their whole relationship is pretty complex. Another con is that there is a brief text scroll that says something like "I don't know why Reo and Mabu are doing the things they're doing, but I can't believe they're actually bad people" around the halfway point in the anime, which comes from the point of view of the "Greek Chorus" character (although this character is also their in-canon daughter, so keep that into consideration too).
In terms of fandom, he's an extremely polarizing character - people usually either love him and think he did nothing wrong since it was all for love, or people hate him. I think it's hard to not be sympathetic to him by the end of the anime due to a combination of an emotional redemption arc and being shown how much he went through to become the person he was in the anime, but at the same time... the show was pretty adamant on saying "Wow this asshole tried to kill kids, and abusing your authority to kill people/isolate people isn't great" in my opinion. That said, this show is pretty abstract/symbolic so YMMV.