Rengoku no Karma is not a manga for everybody. It deals with the heavy topic of suicide and depression, and acts as a warning against committing suicide.

It's a very sad story, and given the premise of infinitely looping to save people, you'll see quite a few people end up in bad ends, all thanks to the main character committing suicide.
This story is great. The High Schooler arc is almost perfect, aside from one small detail I'll save for later.

Now, you may have just noticed I called it an arc. Well, that is because there is a followup to it, covering what I presume to be the last 20 chapters called the "Group Suicide Arc". I did not read it, mostly because my manga site only had the first two chapters of the arc and what I saw was enough.
This kind of story requires only one arc. The High Schooler Arc, on its own, ignoring the small detail, would be what I consider a 10/10. It is thoroughly enjoyable in addition to having a strong conclusion that leaves you satisfied after all the tension and drama it had built up for 20 chapters. A sequel diminishes this fact and makes the story feel less unique, especially when the main character isn't as strong of the original main. The Group Suicide Arc stars a young girl who aspires to be an idol, but isn't cute enough to make it, while the High Schooler Arc features a student dealing with pressure from his parents, bullies, and is betrayed by somebody he was in love with. Which one do you relate to more?

Now, let's not fret over the issues with the second arc I didn't get past the second chapter of. Just shove it out of your mind and put this thought in. Spoilers for the ending of the High Schooler Arc.

Now, at the end, the boy revives, saves one last person and fully exposes the bully and we get an epilogue after what seems to be a rather odd thought from one of the characters, that being "I feel like somebody is staring at me." A year and a half after graduation, the MC uncovers who was watching her, that being one of the people he saved, the cameragal. She proceeds to tell him she hated the other chick and tried to get her bullied into suicide. This whole twist comes out of nowhere with zero foreshadowing outside of "I was a side-character below Bully-kuns fawners", which isn't even true in a meta context. Following that, she asks the MC if he even saved anybody (which he did, having saved SEVEN, possible more people through his actions) with all his effort and he ends up COMMITTING SUICIDE AGAIN. This ruins the main message I got from the story, and just seems like it was done because the author wanted to be edgy. This isn't subversive, this isn't a good twist, this is dull. This makes me feel like I wasted my time.

So, tl;dr, good manga, sleep on the last chapter of the High Schooler Arc and the Group Suicide arc.