I disagree with everyone saying you can`t be evil.
I just finished my 1st playthrough and I set in mind my character would be a relentless, proud and fierce qunari warrior who would grumply follow Cassandra`s orders until he got rid of the mark. Of course, things turned out differently after being beaten so badly by Corypheus and becoming the inquisitor afterwards. Turned out after he only wanted to crush corypheus back, no matter the cost, as much for glory as for revenge...
Altough he ended up saving the day, he did so by being a complete ******* dictator, taking some very costly course of actions with little thought to it, no remorse and no mercy, for friend or foe. In my playthrough, the Inquisitor left a huge fuckin trail of blood and destruction everywhere, helping others only if for convenience. I don`t know what kind of evil you mean, but this is quite enough for me.
It was quite enjoyable. Extreme pragmatism meant complete disregard for it`s own party problems and feelings in dialogue options which turns out unexpectedly cool since some of the guys end toughening up in response to the blunt treatment. I was actually expecting some to leave, but apparently approval is for something else in this Dragon Age. In the end, I think no one liked the Inquisitor, but at least respected him, except for Leiliana, which is a total psycho anyway, and Iron Bull, who apparently can`t get shaken by anything for more than a few minutes. I`ll try another playthrough in the future with a different approach, tough, because not going into the inner circle personal stuff definitely made me lose a lot of content