i think its also important to pay attention to diesels words there at the end too, about his gentleness. i think there's something in the way ein touches him, looks at him, makes him a priority that is new to diesel. yanek saved him from captivity but he never put diesels safety first. i think thats what it comes down to. ein was willing to risk his life to protect diesel. killed unnecessarily, against his instincts, to protect him.
For me, Diesel doesn't care about Yanek. It's hurt me when Diesel with crying face lied to Yanek how important he is for him and in this same he knows that he is in danger and didn't say him nothing about this or try protect him. He also goes to Ein and cheated him.
When you care about someone you try protect him like Ein does it with Diesel. Diesel just doing all bad thing towards Yanek in last few chapters and he cares only about Ein.
Most painfull thing is that Yanek trusted him and cares about him, that's why Diesel betraying hurted him so much.
It definitely was much more than a bit of attraction between Ein and Diesel. With Yanek, it's about Diesel not wanting to be alone to the point that he is willing to risk his life killing of people for Yanek. It's also about him being grateful to someone who saved him from that life in the basement. While with Ein, it's a natural attraction, and eventually started to fall in love with him. Throughout this flashback, Ein was very kind and gentle to Diesel, he cared very much about his well-being and tries to protect him.

There was certainly a little bit of attraction between Ein and Diesel, but right now Diesels actions make little sense. He seemed to care for Yanek a lot, it but yet he chooses Ein. I think, the attraction between them was supposed to have been greater, but it just didn't come through strongly enough to justify what's going on right now.