Telling someone to kill themself is not really any better or worse than any other insult or threat - it's merely voicing the sentiment that the world be better off without you. As such, it is far less insidious and evil than racism. The only time I can see there even being a comparison is if one person was clearly considering suicide, and then someone else urged them to do it - in this case there's a chance that it could push them over the edge into actually comitting the act, in which case I would say it is worse than racism. But that's not what was going on in the post in question.