The layers it adds to the sskk dynamics... How much they have in common... Helping to distinguish what of their behavior was shaped by their upbringing and what wasn't and instead is part of their nature... And most importantly how they would have literally loved each other if it hadn't been for the circumstances. They would have clicked at the first encounter. They would have affably chatted like they were longtime friends. They would have sympathized with the other and genuinely enjoyed the other's presence. AND ALL OF THIS IS CANON
Tbh I think Beast's main theme is that Atsushi and Akutagawa love each other and that any grudge they hold against the other is the consequence of the society they live in no I don't take criticism. What hits hardest - really, hardest - is the fact that even while fighting in Beast, they knew the other had no fault. They knew the other was just doing what they had to do, and it couldn't be any other way. They would have done the same thing in their place, and in a way they were doing exactly that, because their positions - fighting for their little sisters' lives - were nearly identical. As paradoxical as it may sound, even while fighting, they were sympathizing one with the other. The most beautiful yet cruel difference between Beast and canon is that while in canon they fight against each other because there's a difference in their ideals / goals, and can't understand the other's motivations, in Beast they do have such knowledge, they do understand and even go as far as to relate to the other's motivations, yet because of cruel fate they can't evade fighting - despite they would rather not to!!!! Despite actually saying they're sorry for what they're doing!!!! - because the circumstances compelled them to kill each other, a destiny they can't escape. And it's so tragic, and it's so beautiful, and it shows how deeply connected they are, and how much they're made for each other.
