Well, compared to that time, Sauron didn't have his Ring and his Ring contained a lot of his former power.
If Aragorn took the Ring, he would be able to challenge Sauron. Denethor was somewhat of a lesser rank, but still an almost pure Numenorean. The Ring would have turned both of them to Evil, for sure, but not necessarily into servants of Sauron himself. They might just become the new Evil overlords of the world after defeating Sauron.
The main character flaw of Denethor is that he sacrificed his life to the cause of Gondor and Gondor only. Existence and might of Gondor were his main motivations, not preservation of Freedom or Good in the world in general. He was based on a stereotype of a proud medieval or Roman noble, though (untypically) a highly educated one.
If Aragorn took the Ring, he would be able to challenge Sauron. Denethor was somewhat of a lesser rank, but still an almost pure Numenorean. The Ring would have turned both of them to Evil, for sure, but not necessarily into servants of Sauron himself. They might just become the new Evil overlords of the world after defeating Sauron.
The main character flaw of Denethor is that he sacrificed his life to the cause of Gondor and Gondor only. Existence and might of Gondor were his main motivations, not preservation of Freedom or Good in the world in general. He was based on a stereotype of a proud medieval or Roman noble, though (untypically) a highly educated one.