Hot take: Janus does not deserve to be redeemed, and should not be getting the praise he's getting currently.
Like I get it, "Boop noodle go brr" but like... canonically he has manipulated, silenced, and cast aside every one of the sides, he's even manipulated Thomas for fucks sake.
Also! Roman wasn't in the wrong during POF. He laughed at his name, in a time where he was scared and anxious, and didn't have a full idea of what was even going on, and then Janus retorts with a jab at one of Roman's biggest insecurities, knowing it would hurt him immensely. And then Thomas and Patton didn't do shit about it, even after they both heard him talk about the fun house mirror thing- like- wtf- they could've at least been like "That was a bit rude Janus." But no they just fucking stand in silence. I understand that they were kinda stunned, but you would think Patton's dad instincts would kick in.
I apologize I hate Janus very much.
- A Very Angry Roman Stan
I'll admit I'm biased because Janus is one of my favorite sides, so I don't agree with you completely.
That being said, I do think you have a valid point. Janus has done a lot of questionable stuff. We do see a little bit of his perspective as to why he's done these things and I can kind of understand where he's coming from since the other sides were pretty hostile to him and refused to listen, often just on the basis of him being Deceit, even when he was making valid points. Obviously though, that doesn't justify all his actions and I hope that'll be talked about more in canon.
As for how he reacted to Roman's name, I have mixed feelings about that. Like, it was definitely rude on his part but he'd also just revealed something personal so I can understand why he was hurt by Roman's reaction. I know that I personally would probably have said a lot worse. That being said, I don't really fault Roman in that situation either. He was definitely going through a lot and the way Patton (and c!Thomas) switched from "agreeing with Deceit is terrible and makes you a bad person" to "actually he's fine, why aren't you being nicer to him" probably didn't help the situation.
Overall, I think Janus is meant to be more of a morally gray character which is why I don't mind some of his actions as much as I would if they were all presented as being completely and utterly right, if that makes sense? That being said, you're hatred is totally valid! You're allowed to dislike a character as much as you want and interpret them how you'd like.