u/Aromatic_Lobster_113

The only difference is he's charismatic, extremely self assured and far better at hiding it than Niall, and so far the show has focused more on Niall and his struggles, but I think the remaining episodes will show more of this side of Ruben.

The way Ruben acted & spoke in the beginning of E01 at the barn - it's like he was extremely jealous and possessive of Niall - probably at how Niall had managed to find "the happiness that made him click" but WITHOUT Ruben, and that's the part Ruben probably can't accept the most and what TRULY sends him over the edge to beat Niall (even if the scene stops as he probably goes to choke Niall and so far all we know is Ruben is the one that died).
It's the classic "If I can't have him/her then nobody else can".

I don't think Ruben even cared for his own mother that much - but rather what he absolutely HATED is the fact Niall apparently "chose" Alby in the courtroom over him - and now in Niall's own life as well shown by the finality of his wedding with Alby and all it represents.
That's why he wanted Niall to apologize that much in the hospital sequence over Alby.

Ruben is the one truly obssessed with Niall saying they are brothers from another lover, that Niall follows his breathing etc. nevermind all the obvious sexual tension between both of them (Niall at least gets some "release" during all his gay encounters, what about Ruben...?)
Even Alby says Ruben must feel Niall is "part" of him because he was on Niall's first sexual experience, literally guiding Niall into a woman)

In the end, Niall is the only other man in Ruben's entire life who has truly loved and cared for him - and I think that means a lot to Ruben, even in his toxic, obssessive, dominant ways.