Both scenes depict exactly what sets him off. It's the feeling of being trapped and helpless which in turn, causes him to rage and unleash his power.

In Legion, they explicitly tell you that when David feels trapped, his power is unleashed. He destroys his kitchen when he raged

In King Arthur, when they were surrounded and trapped by the King's men, he raged and wielded the power of the sword for the first time.

It's all the same story and never ends well for those who oppose him.