Vaemond Velaryon was a jerk. His jealousy and greed definitely got the better of him.

He was the younger brother of the current Lord of House Velaryon. And with the current heirs being technical bastards, he saw his opportunity to be the next Lord of Driftmark.

But the trouble of accusing a princess of infidelity is a tricky one, but he does so publicly in court. And King Viserys orders his tongue cut out.

But instead Daemon cuts his head off in two pieces. While the dude was not technically wrong, he was just asking for it.