What he saw in the palantir was the vision of Gondor falling and his son dying. His lost time with his sons flew by in a second as the enemy seemed to have guessed what he regretted most- his time as Steward stole more time away from his sons.

Denethor's eyes blurred with unshed tears that this could be the end of his time and lands. How could he let this come to be?

In anger and anguish, he would not bear to let the enemy come and slay him; he would choose his own course of how to die.