Palantirs are inherently evil, and they deceive those who use them. Sauron was deceived into thinking Pippin had the ring, and this led to his demise. Denethor saw what he thought was Sauron's army, and was driven to suicide.

The palentiri aren’t inherently evil. They are mere seeing stones. Sauron’s hubris led him to believe Pippin was the hobbit carrying the ring. Denethor and Saruman were corrupted by Sauron’s influence through the palentiri.