His tyranny is only called such because it was out of character for HIM. By anyone else's standards it was business as usual, and the only evidence that he is the puppet of a Forsaken is the vainglorious boasting of nihilistic villain afflicted with three thousand years of exposure to the True Power, drawn from the same evil as the taint that drives saidin weilders mad. Ba'alzamon's bragging is not a remotely credible source. All things being equal, it is just as likely that HE was the one being bent to the other's will.

As for the issue of ta'veren, it is no different than the special protections other kings get with the divine right and so on. Given that all known ta'veren have been extraordinarily talented individuals it is as likely that his personal qualities resulted in his being picked out for greatness as the candidate who could best serve whatever purpose the Pattern had in mind (IMO, uniting humanity to forestall the Trolloc Wars 2), as that his achievements are based solely on some superpowered boost.

Actually, the ta'veren factor works even more to his credit, because the expansionist conqueror seldom makes for a good ruler. As we know from the books however, a ta'veren is driven along his course and must achieve what the Pattern wants him to or die. Hawkwing, according to his biography, was a good ruler who was content to stay at home and tend his nation, but was driven to conquest by necessity, implicitly by the mandate of the Pattern, and explicitly by the hostility of the White Tower.