Not only that when I went to report him I was greeted with "It's not a big deal"
I think i see your problem, the report feature does not require you to talk to other players while reporting. You are not even supposed to mention the fact that you are reporting. You mute, make a note of it (for after the game), finish the game, and then report.
Hell to request others to report is reportable in it self, it is considered harassment by riot.
My 2 cents is that telling someone to kill themselves is no worse or better than telling them to fuck off. Let be honest here people have hinted at suicide for a long time, ever heard of taking a long walk off a short pier? You are not being asked to go for a swim. It has been on our culture for a long time, people are just being blunt about it for the shock factor.
No one really expects them a person to do these acts, Its just an extreme way of telling someone to go away. It is time for people to grow up and stop pretending that they are really upset about it.
Yes it is against the rules. Yes you can report them. No you do not have to make threads about it or preach in game about the problem.