DP #2 - Boundary Issues in the White City. In a classic example of transference, Denethor attempted to negate his feelings of inadequacy (caused by his inability to save either his wife or his city from wasting away under the Shadow of the East and the fact that a Steward is NOT a King) by projecting the negative aspects of himself onto his son, Faramir, where they could be easily faced and punished. Torturing Faramir as worthless (a scapegoat, if you will) somehow alleviated Denethor's own staggering sense of guilt and helplessness.
Also suffering was Boromir who was denied his basic, fallible humanity by representing all of Denethor's nobler aspects and virtues. That he did not become a completely amoral bastard due to Denethor's idolization beyond all reason is a major point in Boromir's favor.
One could say that the Steward did not see either son as individual beings at all but rather the two halves of the whole that was Denethor. Since he perceived them as only extensions of himself both, especially Faramir, were expendable.
In conclusion, Denethor was an asshole.