I think that's because the killer deliberately pushed him to act by noticeably making an appearance in his private life (that picture he took with glasses guy). To me, it seemed like he realized by then he wouldn't be getting away by simply ignoring him and chose the "best defense is offense" approach instead.
And you're right, this actually also made me think of Black Mirror! :)
Makes sense.... though it was shown that he thought it might have been a coincidence (now because of this I'm wondering if he left the guy because he was angry or because he wanted to avoid getting him dragged into it... pretty sure it's the former and hopefully he left him for good; I know wishful thinking )
Oh, I wouldn't worry too much about that either way... I mean - that guy's getting killed off one way or another, without the shadow of a doubt ╮(￣▽￣)╭
This seems interesting.. I'm kinda reminded of black mirror due to both seme being authors XD The only thing I don't understand is his sudden motivation to report him in. Earlier it was just self preservation but now it seems like it's gone a bit beyond that all of a sudden...(probably so that he won't get killed in the end I know)