they gotta progress the plot somehow
normal for a cuckoo. the love probably lesser than the dependency and obsession. it's also possible that in the spur of despair and anger he impulsively told the stalker. it might be not so much of a planned murder. yeah although all this time he gave me this charismatic psychopath child genius vibe, he might actually not as smart as our aqutan. also it seems the car accident wasn't his fault? although he's indirectly the cause but he didn't seem to know.
I actually think it makes sense , Kamiki is insane from all the abuse, he attached himself to Ai , so something as "dumb" as breakig up destroyed him, a lot of people torture/kill their exes lime that too is a sad reality.
Oh yeah absolutely normal to kill the love of ur life just cause she dont want u no more