Cultural thing I guess. In my region, people easily using the i’ll kill you expression in conversation, without the intent to really kill ofc. I’ve seen the phrase is being used a lot in Japan and South Korea as well. In this manga, the phrase represent possessiveness, so I guess it could mean somewhat romantic.
I really get the feeling that it is serious in this case.
As for other times, people say it all the time without actually meaning murder. So it just mimics reality.
why do authors take ''ill kill you'' sentence so lightly? seriously im curious