Well, suppose there's a group of people saying that another group of people (which happens to include you) should be killed as soon as possible. In an ideal world, they probably wouldn't do that because they'd recognize that collective aggression against others could rebound on them, but in the imperfect world we actually occupy it happens all the time. Of course, you could just figure they'll limit their social opportunities and eventually grow out of it or develop some higher wisdom, but just how long should you have to put up with threats from random strangers and anxieties about your personal safety while you are waiting for your antagonists to become enlightened?
Some people think it is. In many legal jurisdictions, prosecutors and cops rely on the idea of a 'true threat' which has to be specific and imminent. So a generalized expression of hate like 'all _____ should die' wouldn't count. Here's a relevant example, about a woman in Michigan who eventually resorted to shaming her neighbor int he media because she had no confidence in reporting to police:
The problem with a very mechanistic/binary approach to legal questions is that it leaves a lot of room for antagonistic or outright threatening behavior that nevertheless remains within the bounds of legality. Experience suggests that's likely better than vague or arbitrary standards, but it's not a simple issue.
