As students do, once their course was over, they moved on
For the sake of his family, Drona goes to Drupada as he believed his Childhood friend would take care of him
There he is welcomed by Bheeshma and is made the teacher for both the Kauravas and the Pandavas.
Drona then let's Drupada go but takes away, half his Kingdom. Vengeance achieved
His Son Drishtadyumna then kind of illegally kills Drona, while he is meditating about the "death of his Son".
End of Vengeance 2.
This is thankfully stopped by Lord Krishna, who then curses Ashwttama to roam around in forests, with blood and pus, for eternity as death won't come to him
He easily could have done it. He was a king. But he didn't. And his entire lineage paid the price.
Drona too could have let go of the hatred. But he didn't.
All for Vengeance.