That is exactly the psychological trap Kane set for himself. Finishing that review was the ultimate act of arrogant self-sabotage.
By completing Leland’s scathing critique, Kane was trying to "win" on three different fronts:
Proving He’s Still a "Newspaperman": You hit the nail on the head regarding the Declaration of Principles. By writing a honest, brutal review of his own wife, Kane was performing a perverted version of integrity. He wanted to prove to Leland—the man who kept the original document—that he could still be an "honest" editor, even if it meant humiliating Susan to do it.
The Ultimate "Flex":
Kane wanted to show Leland that he could write a "Leland review" better than Leland himself. It was a power move designed to say: "I don't need your moral guidance; I am the master of the truth, even when it hurts me."
The "Firing" Paradox
He fired Leland immediately after finishing the piece. This proved that Kane didn't care about the content of the review—he cared about who held the power. He could tell the truth about Susan because he owned the paper, but he wouldn't allow a subordinate like Leland to hold that same power over him.
As Leland later tells the reporter, Kane "always typed his own headlines." He would rather destroy his wife’s spirit than admit that someone else had a higher moral ground than he did

I agree with you but also remember this piece.
[On Kane finishing Leland's bad review of Susan's opera singing]
Mr. Bernstein: Everybody knows that story, Mr. Leland. But why did he do it? How could a man write a notice like that?
Jedediah Leland: You just don't know Charlie. He thought that by finishing that notice he could show me he was an honest man. He was always trying to prove something. The whole thing about Susie being an opera singer, that was trying to prove something. You know what the headline was the day before the election, "Candidate Kane found in love nest with quote, singer, unquote." He was gonna take the quotes off the singer.
http://www.imdb.com/board/10033467/quotes?item=qt0259161
IF YOU WORK FOR A LIVING WHY DO YOU KILL YOURSELF WORKING

Do you remember the Declaration of Principles?
I think Leland angered Kane by asking to keep the document they were written on so Kane felt is necessary to arrogantly prove to Leland (and to himself) that he still possessed principles.