Words actually matter. You might mean, "Get over yourself," or whatever... but you literally commanded SUICIDE instead.
Even though it's a trendy acronym on the internet, and in gaming communities in particular, if you stop to really think about the idea of telling a complete stranger they should murder themselves is really really fucked up.
Riot has made it no secret that doing something like this... wishing or threatening harm on others... is an egregious infraction against their philosophy and policy. And for good reason.
I'd like to take it a step beyond saying, "Oh, well now you know not to say that in LoL." Genuinely, please consider what you're really saying with something like "kys" and avoid using it in the future... anywhere.