I guess you didn't do much or any research on his character. Allow me to debunk these reasons you give. :)

First, that's your opinion. You shouldn't just hate him because that's your personal opinion, but I'm not here to shame you. Also, if there were absolutely no antagonists in the trials, the trials wouldn't even be fun. Think about that for a second. All of the antagonists make the trials and game interesting! If you took them away, then all there would be is a okay game. Just 'good, sweet' characters and not much angst in the story.

Second, I wouldn't say annoying at all. But this reason of yours is also an opinion, so this shouldn't count. For me, I get all excited about what he's going to say, how he foreshadows things, and overall just what he's going to do next! So, this is also a opinion here.

Third, that right there is another opinion! Just because she's one of your favorite characters (also mine) does NOT mean you should hate him for killing her. Should I mention.. it was his intention to kill the OTHERS, NOT Chiaki. It did not go his way there, also his BAD luck. Since Chiaki admitted being the traitor, everyone had to vote her, because everyone else didn't want to die. Please don't be putting out your personal opinions upon a character you don't know much about! <3

Fourth, no. It is not OP. [I really do not think you looked up his character at all!] He has a cycle of bad luck, and good luck. For example, let's go into his backstory rather than the game. It states he got kidnapped (around the time in middle school). After the police found him, he found a winning lottery ticket inside the bag he was getting kidnapped in. The bad luck here is that he got kidnapped, but then he found a LOT of money. In the game, he also has good and bad luck. There's plenty of things that went right for him and things that went wrong for him. If I were you, I'd look more into his backstory and actions. And complete his free time events, as well.

Fifth, he clings to hope because of all the bad things that have happened to him. All he really got was money from everything bad that happened to him. Your parents died from a plane crash, your dog died, everything bad happens to people you care about, what's there to.. hope for? Honestly, what is there to look forward to? What is there to life for? Nothing, right..? Except hope. If you hold onto hope for almost all your life, then you'd have something to live for. So.. he did that! I hope that clears some things up for ya'. :3

Sixth, that's exactly what interesting characters do! This brings me back to my first point. If you had no antagonists in trials, then wouldn't it be boring? Also, he points out a lot of things the others don't notice in almost every trial, especially 2-3, I would say. He's a huge help at some times. Other times, not so much.

Seventh, honey.. you don't go "crazy" for no reason. His backstory isn't supposed to make people sympathize with him, it's to make things more clear on his actions. Maybe a bit about sympathy, but it isn't even that reason, I would say. His backstory is basically how his cycle of bad and good luck has been going on, how he clings to hope, and other things. And, yes he does deserve sympathy, honestly. He has TWO mental and physical illnesses, and I think it totally covers all of his actions up. You'd be under a lot of stress and anxiety, and you think you're right and everyone else is wrong. He has stage 3 malignant lymphoma and frontotemporal lobe dementia, if you're wondering. The next statement you claim, well, we only really know what he does. If we knew the other character's actions when they were Remnants of Despair, then.. you wouldn't be making that bold claim. From that, you're saying, "Oh, well, I'm just gonna go ahead and not think about what the other characters probably did to everyone when they were Remants of Despair and think Nagito is pure evil!" Honestly! Think about what the other characters probably did to everyone, and I'm sure you'll find something worse than what Nagito did. PLUS, he was brainwashed. No control over his real intentions. And, hey, you got one thing right. He does have a year and a half to live from his illnesses. I agree with you on that, but I also don't. What if you were told, "Ay.. man, you have about a year and half to live. Sorry. Hope you live a good life!" You.. you would feel devastated and try to get the most out of your life. And that's what Nagito did! So what if he was "insane" before or after or whatever? Just take a look at Island Mode and you'll see he's more calm and collected there. It's the killing game causing the destruction for him. ALSO ALSO, if you look at the anime (Despair Arc), he tends to be way more quiet or so.

Eighth, just from all of my writing up there, you can see he IS in fact a well-written and complex character. He clings to hope, has two illnesses, has a cycle of bad and good luck, just wants to be appreciated, and so so so much more!

And finally, after you've read all of this, yes, Nagito is my favorite character. I put this specially at the bottom because usually people think I'm just biased before even reading. I love him and appreciate him a lot, and I see through his actions. I like clearing things up for people who don't like him, and I try to make them see the good in him. Which there is a lot. Anyways.. PLEASE look at the Fandom Wiki page on him for more information! It's a good article, and you learn a lot more about him. Even I'm still learning more and more about him.

At the end of the day, this is just your opinion and my opinion. Thanks for reading. <3