It isn't amazing. It's childish. It's something an internet troll would do, not someone who has a pretty legit shot at being POTUS. As for what his tweet means, it means Trump is basically trying to incite violence and someone is gonna end up dead before too long. It also means Trump is unfit to be a public servant. That's all.
And before anyone gets their panties in a wad, I know I've joked about Trump and made fun of him a bit, but that was all funny ha ha and I didn't have a huge problem with him (and frankly I rather enjoyed what he did to Jeb, Rand, Cruz, Marco, Samuel L Jackson, the media, et al). This shit, on the other hand, is retarded and dangerous. I get that Trump is a "loose cannon" and not "politically correct", but the dude needs to start acting like the front runner he is and tell everyone to chill the fuck out.