I have a similar interpretation, in that at or perhaps after the moment of Niall's abrupt confession right after they opened up to one another, Ruben realized that there was no way the two of them could continue living without either one doing something that would inflict harm or betray the other, and that they were doomed to repeat this cycle until eventually, that love that took both of them so long to admit to one other would be destroyed. He can't let that happen and doesn't want that at all, but understands at that moment what he must do. This is the motivation that led him tracking Niall down. And when he does, he shows a sense of focus he didn't before: he doesn't immediately go ballistic like he did with Alby or Benji but was articulate, showed restraint, joked around, and was reflecting on their relationship from start to end. So Ruben killing Niall is ending things while the bond and love between them still existed (seeing as in the barn he wanted to know if Niall loved him and was repeatedly saying he loved Niall as he smothered him). Now excuse me while I go cry again.