
spoiler....
From what i read, i can give you a rough explanation
Past, seme was a transfer student, so the bullies in his class bullied him, of course. Uke became friends with seme, since they met before as well (actually, the scar in uke's face was from when he tried to protect seme in kindergarten or something). Long story short, uke tried to defend Seme from the bullies, and soon he became their target or something like that- they continued to bully Uke, and seme tried to defend him (seme was a lot crazy, so it wasn't actually defence) by letting the bullies hit him instead. Uke felt guilty, so every time he tried to apologise to Seme, Seme looked blankly at him and said, "I don't care". This went on, and Uke started getting more and more depressed. Seme lost his part-time job because of uke, and he let the bullies hit him because of uke. And whenever Uke apologised or asked him why he did it, he always said "it doesn't matter". Thus, uke got more and more depressed and guilty, and a lot of things happened. One day, when the bully approached them again, this time uke defended the seme and beat up the bullies, and the seme was happy. Uke got more mentally insane and depressed, so he started convincing himself, "yeah, he is right. It doesn't matter". So conclusion (it's my thinking) Uke got so mentally stirred, he was the one who killed the bullies, not the seme(its my hypothesis) the seme just convinced him, that the seme was the one who killed the bully.( ̄∇ ̄"). It's 100 per cent that the seme is 100 per cent crazy, but the uke is also crazy.
PS- s2 is all past related, so its hard to explain every thing.

when it first published, i just read raws, praying someone would finally upload this amazing manga but no one did, so I only read the raws till S2. and now I am rereading it and still am not bored.

spoiler- in the last chapter, Uke tells him he is not in love with him and ends it with seme. Seme is a psychopath so he cracks on hearing that uke doesn't love him- He rapes him and writes down his name on uke's abdomen with glass and says that he loves him and uke loves him back. and season 1 ends at ch 37 or 39, i guess.
if you read the raws, reading all of season 2 will give you a clear understanding of their past and then boom, the last chapter of S2 will take back all the understanding. its awesome how till the very last chapter of S2, no one could figure out what was actually going on. the last chapter of S2 will take you back to the chapter 1.