Why Smear Yovanovitch?

Why did tRump smear Ambassador Yovanovitch? He had the power to replace her at any time and without explanation. The smearing was unnecessary and seemingly pointless. But not, I think, for tRump. His motivation isn't simply that he's grifter. There's a second key: he savors the cruelty. That's half of every decision he makes. It must be cruel.