Reprehensible behavior, I agree. Reveling in the misery of other people is not something that serves us as a civilized nation. It's something all people are capable of doing. But, it's a choice.
Trump's supporters use it as a campaign strategy and rallying cry.
Who’s reveling in someone else’s misery? All that slogan says is that Trump supporters don’t care about your feelings, whether negative or positive. It’s not specific to misery. If you’re miserable, that’s on you. Stop being miserable and that slogan will stop being offensive.
In other words, get on board with the direction we’re heading and you too can be positive and enjoy life.