I think that he was mentally unstable and unable to govern and to take firm political stand on any issue.
He was an older brother of the Czar, he didn't become a Russian ruler for a reason.
For sure, mental aberrations of Constantine, for I would rather speak of them as aberrations than instability, were a hereditary trait due to his father Paul I and grandfather Peter III. Moreover, Constantine had clearly realized all perils of his potential reign long before the death of his elder brother Alexander I; that was the first cause of his reluctance to take the crown. No doubts he would share the fate of his father and grandfather had he dared to do it.
Though, his Polonophilia serves as clear evidence that his mind was deranged to a certain extent.