...

His friend better be a good guy or I will have to kill the thrn former friend ┑(￣Д ￣)┍

agreed

well, he didn't have to walk away.. So Sempai repeated a rumor everyone else is saying. Couldn't he have just said 'that's not true, he's my friend and we've talked about it'? But no, he walks away on his LOVER/friend over a trivial statement.. I think that was over the top sh*tty..

Well I mean they would have walked away from each other in silence anyway, considering this kind of romance manga, if Sendai really wanted to continue the conversation, he could have grabbed his hand and went like "I'm sorry for insulting him, stay with me?"
Words hurt, if someone said something bad about a friend even if it's trivial, it's still untrue and rude. He has the right to walk away （￣へ￣）

senpai*

But he doesn't know of his friend is really a not a player. He only saw him in the roof
Other plus they never talked about that. Plus Senpai didnt just repeat he says is agreeing with them and saying he looks down on him.
I think he walked off because he was mad and disappointed him his senpai not to just be a brat.

sure, I agree, he is repeating a rumor because he's jealous. Of course he didn't know the truth, neither of them do, it was bad choice, but still. Consider you are intimate with someone, their lover and friend.. you say something , not a horrible thing, just a rumor..about someone because you feel threatened and your best friend/lover defends them by getting p*ssed at you and leaving you standing there. That is so much of an over reaction that I would believe my lover was having feelings for that other person.. it would hurt my feelings pretty bad.

That I understand completely. But also he is 100% knee to this first friend and first lover.
I don't think he is honestly really thinking clearly I know felt bad because he came back after he walked away. Just senpai disappeared