Right here.Funnily enough, the first post reads like it could have been written by me. Pretty much everything I have ever said or felt about this character is accounted for.
This is the bit that most got me:
His family treats him like a second class citizen, he has very few friends (unless I am forgetting someone, only Toru... and even Toru called Natsuno his best friend even though he was new in the town and Masao and him were friends for years...that must have hurt)Hooooly fuck yes! It's so obvious to me that this is the main reason Masao doesn't like Natsuno. There may be other factors in play (certainly neither of their behavior helps matters) but that's probably the biggest driving factor--Masao is most likely terrified that Toru will abandon him in favor of someone more interesting or popular. And ultimately that's exactly what happens.
Losing a friend is always painful, but its very different to lose a friend when you have +1000 friends than losing a friend when you can count your friends on one hand...And that's almost certainly how Masao sees it too. Whether or not this reflects the objective reality, or lines up with how Toru sees it, is a different matter, but it's beside the point.
I will say that the poster seems to be very anime-focused. They go on about how innocent Masao is and how he never actually wants to hurt anyone...all of which is arguably true in the anime, but in the manga he's a lot more openly villainous. They also posit that his family has always treated him as the unfavorite, whereas the manga makes it clear that he used to be something of a favorite before his nephew came along. They're not wrong for zeroing in on the anime, since Masao isn't a completely consistent character between the anime and manga; but it's still a bit weird, especially since this is a website primarily for manga fans.
I also find it interesting that the people who attempted to argue with this poster all seemed to say something to the effect of "Well why didn't he try to fit in better?" or "Well why didn't he just TELL Toru about his inferiority complexes?" or "He still deserved what he got for not trying to grow and improve!" (as if most of "Shiki's" cast improves in any really major way, rather than just getting worse). Did it never occur to these people that maybe he can't? The series makes it pretty clear that when he says something obnoxious, he tends not to know why the other characters find it obnoxious in the first place. I'm surprised nobody suggested he might have some sort of disability, because that's been my theory for a long time (I'm disabled, and he reminds me of myself from a time when I was a stupid kid who didn't know any better).
But still, it's good to see that something new has been written about him since I last went looking. It feels as though every time he appears on Tumblr in a non-bashing context is from when I first ventured into the fandom (that is, three some years ago)!