He’s just baiting Trump, toying with his emotions and ensuring his anger is revved up deep into the red zone. Angry people make bad decisions and self-defeating ones at that. As Hillary said, a man who can be baited with a tweet…
Considering he’s already hellbent angry about Mueller and his damned questions, the leaks about his congratulatory phone-call to Putin, and the three lawsuits with women he’s harassed or slept with, this certainly can’t help his fragile emotional state.
Also, this is the type of language Trump’s base responds to. It’s about dominance. Trump can’t let this go without looking bad in front of his tribe. And I think Biden’s more than aware of this.