It hurt to watch it. It's shockingly good, but it's a tragedy that grows out of absolutely everyone's self-destructive and thoughtlessly cruel choices. It's beautiful in the purity of its dramatic construction, but don't expect it to be fun.
It's based on a pornographic computer game in the harem genre. In it, you played Makoto Ito, a high school student who's popular with the girls! Most of the play routes ended conventionally, but the game became notorious for a few bad endings (neck-slitting, suicide, etc.)
Naturally the anime producers went dark. I'd heard about this and it sounded cool. What I hadn't known in advance is that the show really, really earns its bad ending. The most shocking scene in ep.12 doesn't have a drop of blood. The worst thing is that it's all so true to life. People behave like that every day. Makoto is a fairly normal teenage boy, in most respects genuinely helpful and even nice. He's popular. His interest in girls is basically focused on getting them into bed, but he's hardly the first boy of which that's true. He's willing to do the expected boyfriend stuff if that's his best chance of getting his leg over, but he's not keen on long-term relationship stuff. He's a bit flighty.
At the start of the anime, he's an innocent. His view of Kotonoha is sweet. However when Sekai starts pushing them together, he's always coming on too strong, kissing and groping Kotonoha before she's even remotely ready. The worst of it, though, is that he doesn't see it. He's blind to her feelings. He's not even really capable of caring what she wants and his apologies afterwards are basically just another tactic.
So you're cringing. It's deeply uncomfortable to watch their dates. It's like an animated textbook on How Not To Treat Your Girlfriend. Then, at last, he has a nice, normal afternoon hanging out with her and her little sister, for once acting like a civilised human being... and it makes him tired. It makes him want to go somewhere else.
So he's a cock. What makes him particularly damaging to others, though, is his inability to choose and communicate. When two-timing Girl A with Girl B, he refuses to cut it off officially with the former even when his behaviour towards her is cold enough to freeze nitrogen. You've practically got to put a gun to his head to make him come clean with someone and dump them. Even when that person is clearly suffering and clinging to the idea that she's his girlfriend, he doesn't see any moral angle to his behaviour.
In fairness, it's hard to imagine many girls sticking with someone who behaved as coldly as he does to Kotonoha. In real life, he'd get dumped sharpish and it would all be okay. However Kotonoha isn't that kind of girl and his actions are shaving away at her sanity.
All that said, though, he's still human. If he were simply a monster, this show wouldn't have the power it has. Very, very occasionally he shows some self-awareness. "I'm the worst." Towards the end, he does appear to show guilt and a desire to make amends, although by that point it's impossible to know whether this is genuine or just another commitment-avoiding tactic. (Probably a mixture of both.) It's also likely that none of this would have happened if Sekai hadn't effectively pushed him into it with her weird matchmaking and coaching. To what extent was he being shaped by his opportunities? The Matoko of ep.12 is barely recognisable from the Matoko of ep.1, although I wouldn't call the latter particularly likeable either.
That's Makoto. He's the cause of his own downfall, but the girls in some ways reflect his flaws and play equally important roles in the unfolding of the tragedy.
Makoto can't communicate, you see. Well, that's Sekai. She liked Makoto from the beginning, but a trivial misunderstanding in ep.1 made her turn matchmaker for him and Kotonoha. After that, her training sessions... no, no, no. Your heart bleeds for her, but no. If she'd just been open with him from the beginning, all this might have been avoided. (And, painfully, someone else has been doing the same for her.)
Kotonoha is shy, insecure and more than a bit fragile. Things keep happening faster than she can accept or process. She's bullied at school, she never puts herself first and her tragedy wouldn't have happened if only she'd been capable of being a bit more assertive. She's capable of blaming herself for Makoto's behaviour. Again, no, no, no.
It's a fascinating series. It's deconstructing the harem genre into horrific, bloody pieces. Ep.1 looks a lot like the usual generic fluff, while it's interesting to note that the traditional Lecherous Idiot Best Friend is for once far more likeable and straightforward than the main character. Taisuke is at least openly girl-crazed. Makoto will keep pretending to be your boyfriend even if you've got photographic evidence that he's sleeping around. (Mind you, apparently Taisuke's a lot less nice in the sequel game/manga, Cross Days. I hear that Makoto also manages to be more loathsome in the Cross Days manga, which scares me. Meanwhile the School Days manga is supposedly more family friendly than the anime, but almost more disturbing by being nicer and subtler about it. Makoto's nicer in the original game, too.)
These people don't necessarily think they're doing wrong. Makoto probably qualifies for "evil", but there's no malice in him. There's no forgiving his frustrated, infantile anger at Sekai for SPOILER, but he never does anything as cold-bloodedly and premeditatedly vile as what those three girls at school did during the culture festival in ep.10. They have no redeeming features at all. They get away scot free with everything they do.
I admire this show. It's ironic and amazing that an artistic statement this lacerating about human nature and sexual relationships is an adaptation of pornography. It's the evil soul-destroying opposite of wish fulfilment and randy idiot teenagers should be forced to watch it. Makoto richly earns what happens to him, but it hurts just as much to watch, for instance, the girls who made the situation happen with the best of intentions by putting their feelings aside for the sake of someone else. There's so much horror here. Note, for instance, that Sekai isn't insane at the end. It would be so much simpler if she were.
It's draining, it's ugly and I think it's a masterpiece. However it's also twelve episodes of teenagers of variable intelligence destroying the lives of themselves and others, so there's a good chance that you'll hate them and the experience of watching them. Approach with caution.