The palantiri were created by Fëanor. The kinslayer whose pride, rage, and desire for vengeance drove most of his people to their doom. The potential to corrupt was always present in them.

In the LotR, Aragorn bends a palantir to his will and uses it for good with great difficulty. He manages to do that, because he is (in addition to everything else) the trueborn king and the palantiri are his birthright. Denethor, on the other hand, succumbs to corruption. While he is a powerful lord with good intentions, he is only a steward, not a king. The right to use the palantiri is not inherent in his being, because he only wields power in someone else's name.