i wish there was a story about this sempai! i want to see him gain happiness too :(
I don't think that what he did was that grave. He didn't put gun on her head and said 'if you don't leave Ryousuke I will blow your brains out'. Yeah he said that that dude still likes her but everything after that was on her. If she really loved seme she wouldn't have left that easily. Honestly, I think he saved seme from future heartbreak cause if she was ready to leave him then, she could've easily done it in the future as well.
I don't think it was an unforgivable mistake but i also dont think it was something good. Ofc the girl is also to blame, but had Akio encouraged her a bit, she would stay with Ryousuke, and very likely soon forget about her ex-boyfriend and keep living happily with him (bc people do move on from past loves). Akio knowingly did that exactly to avoid that future where she would never leave, which is not something a friend would do.
I really liked how Akio didn't forgive himself as easily as Ryousuke did. After all, he did something bad to someone he claimed to love. I think he deserves happiness too, but the fact he realizes his bad actions and regrets them is quite refreshing in Yaoi.
I feel bad for Senpai tho. He liked Akio but gave him up so he could be with the person he truly loved - an interesting contrast to Akio himself. Wish he would have his own story with a happy ending (=・ω・=)