I've gone back and forth like this with my opinions about MP100 in general, especially when rewatching the anime. I too often feel like I ignore things that I think are wrong, and am only seeing the good in people and story because I like them and I fear that clouds my judgement. However, I believe this feeling and mindset is incorrect, and it represents our fear to believe in the good in people and being afraid to actually hope and believe in "happy endings" being really possible.
Reigen is flawed and does bad things sometimes, but so does everyone. The proof of someone's true character, I believe, lies in looking at all their actions as a whole. Many times Reigen's actions can be argued to only appear good and there are some reasonable explanations to claim his intentions were actually self-serving. But again this can always be done about anyone or anything to try and skew what happened from a distorted perspective in order to fit with what people want to believe. However sometimes people do things that are simply undeniably actions of bravery and kindness. And at the end of the day, we have to ask ourselves what actions are more important and meaningful. What defines someone's character more: Giving someone a job and calling them a friend, or what kind of facial expressions they have at certain times?
In the end, it's not about "either they are good or bad" when it comes to people and their actions. It's always more complicated than that when it comes to judging people. It's unfair and incorrect to make judgements about someone and about situations based on other things that person has done. The two matters are seperate and unrelated. "That is one thing, this is something else." Reigen is a good person who cares about people, even though he may not show it all the time. Serizawa has been taken advantage of by someone in the past and used by them, but that doesn't mean it will happen again to him with other people like Reigen.
We have to trust in people sometimes and believe things will work out even though we don't know for certain that things will go the way we want, because that's how it always is in life. Serizawa believed nobody would ever need him or want to be his friend because of the way he is and because of his actions in the past. He believed that what happened in the past would always happen, that he would always be anxious and hurt people and could never change, but he was wrong. I think it's equally wrong to judge Reigen or anyone as "bad" or "beyond hope" because of some of their past actions and mistakes, and it's the same kind of self-defeating mindset that Serizawa was plagued with.