This is based from my own understanding..

Why is everybody confused? The plot is easy to understand. The ML and MC were childhood friends. The kid MC was subconsciously possessive to kid ML, well, firstly because the MC likes the ML but I think the reason may be because he's scared that the MC might get abandoned by the ML—which is why he pushed away the little girl because she might be the reason the ML will abandon the MC, he was threatened in simple words. BUTT The ML wasn't, wouldn't AND won't do that at all but he kept it lowk for some reason (cause he freaky like that). And then here comes the promise, the promise which where the ML's obsession towards the MC had begun. Their promise was to not let anyone take one of them away and fall apart (Though, I think it was just a one sided promise..). And to make that happen, the ML has to isolate the MC from everyone (including himself) in order to not make anyone take the MC away and make the MC be dependable to him (It's a classic move from crazed obsessed characters to do with their partners..) This is all I can think about and I hope this helps you understand You can also add more in the replies if you have something (人 •ᴗ•)