I hardly think that killing ONE MYSTIC qualifies as a "bad ruler." If you look in the novel, Sun Ce was actually quite the opposite of arrogant - just somewhat paranoid. After all, if you look at the novel, there are many people who rush to join Sun Ce of their own free will, such as Zhou Tai, Jiang Qin, Chen Wu, Dong Xi, Yu Fan, and Ling Cao.
I mean, honestly. The number of "dead innocents" to Sun Ce's credit is one. You can't call him tyrannical simply because of one execution. Cao Cao executed Kong Rong and many, MANY others and killed the commonfolk of Xu. Liu Bei executed Zhong Yu and executed Yang Huai and Gao Pei - who came to say goodbye to him, not kill him - when he could have easily allowed them to live. Sun Ce - who treats the peasants well and attracts new talents - kills one mystic and suddenly he's egotistical.
"There are those who try to shape the world to their own whim,
and then there are those who allow the world to shape them.
It is in the balance that greatness is achieved."