Bro, that's a flashback; Aku is one of the guys killed in the last arc (he was poisoned).
Also grandma already said that she will go to hell with her so she will probably help with Spica's vigilante hobby.
He didn't choose her; like the other comment said, he did it out of guilt.
Also, even if she is his girlfriend, there was never an option; the guy practically only sees her as a good friend and only has eyes for Ria.
The audacity to tell me that I seem to haven't read the chapter when you look...