It's a one shot and its about an uke who is an assassin and was hired to kill the seme who is also an assassin. The uke failed to kill him because the seme is more stronger. And instead of killing the uke, he hold him captive and raped him because he took a liking to the uke. There is also a sequel but don't remember much of it.

If it familiar please let me know because this manga won't leave my mind.

And sorry if my grammar is bad
I'm bad at English.

I think it’s one of two works by MURAKAMI Maki but it looks like they were removed from the sites I use so can’t bring up both titles. They were both about the same pair but one seemed like a fleshed out version of the other, and was conclusive.

I could only find the title of one of them: Koroshiya-san no Shippai

I think the English versions were removed so now I don’t know where to find/read it. If you find it, lmk.