the craziest part about the whole episode is that niall admitted to hooking up with mona and fathering ruben's kid right after he also admitted to being gay. we as the audience knew that he just hooked up with mona to get back at ruben, but ruben is discovering that his brother betrayed his trust by hooking up with his wife, his child isn't actually his, the only reason his brother did this was to get back at him because his brother isn't even into women, and he went to prison & therefore missed his mother's death for beating up the wrong guy, all at the same time, right after he confessed some deeply personal info about himself and the abuse he experienced.
it absolutely makes sense to me why this would evoke a much deeper reaction and sense of betrayal than even when ruben was sent to prison the first time. while ruben made threats to niall that first time because of his 'betrayal', he moved on and didnt carry through with them, even when he was still bitter. while this time he has also lost everything in his life -- his mother, his child, mona, and even niall again because of his betrayal. and as much as ruben claims to take responsibility for his actions in this episode, we also see how he continues to engage in the same behaviors and continually blames others for provoking his violence, just the same as he did when he was young. so of course this time he will take it further, will actually try to kill niall, because ruben can't accept that his own actions have led to his life being shitty, and just like niall has been blaming his internal homophobia and his shitty life on ruben, ruben blames his violence and his shitty life on niall and niall's wrongdoings towards him.
i also noticed how when ruben confesses how his father sexually abused him, and how he had physical reactions to it and had complex feelings surrounding it, niall kinda withdrew from him. and it really brought to mind how ruben kinda did something similar to niall in a way, by basically forcing him to have sex with mona, and later on in the hospital, and at several points by groping him and just general borderline or straight up sexual harassment. neither of them mention this but i think that is a big reason why niall ends up not wanting to hear about everything -- not because he is dismissive of sexual assault, but because ruben's words bring to mind the same things he feels about ruben. and even when niall confesses being gay, he still will not vocalize how he feels about ruben, that is still the boundary that cannot be crossed, until it is finally crossed in the last moments of the show, and when niall is the ultimate victim of ruben's violence.
and while i dont think niall wanted to die per se, he is willing to walk into that barn with ruben likely knowing what will happen to him. even when he could have taken so many other steps to diffuse the situation, like calling the cops, or just not going into the barn with ruben, he always was going to do what ruben wanted. he is reckless, because he craves rubens presence and attention even when he should stay away. like niall said, it's a chemical addiction; even when niall has been away from him for years and hasn't chased him down like he did in episode 4, he is drawn right back in, and he can't resist ruben.
we can also see that just like ruben, niall has not really changed, even as he has. although he may have finally come to terms with being gay, and is getting married to a man, he still looks miserable, even before he finds out about ruben coming to the wedding. so in the end, they are back at the beginning: both niall and ruben are tortured by their own minds and unable to escape the cycle, until eventually ruben externalizes that violence like he always does, in a way he can't take back. niall says in episode 4 that he died a long time ago. i think this is true, but it is also true, probably even more so, for ruben. so their deaths at the end of the show is just an externalization of that inner death, and an outer culmination of the cycle of violence that has already destroyed them.