That's simply a recognition that looting begets violence, meaning that the looting must be stopped before things become more violent, and a call to action to those who appear content to let the looting run rampant as if it's somehow OK or justified.
In such an escalation both protestors and counter protestors would be shooting, so bringing the situation under control quickly is in the best interest of the protestors and the community as a whole.
Trump's tweet was just a recognition of that.