I think you're focusing too much on the slavery issue here. It is entirely possible to play the game without owning a single slave, so I don't think slapping the "slave master, ergo evil" label on the player is appropriate. Furthermore, it is entirely possible to fit the D&D textbook definition of "good" and still be an "upfront some-what aggressive personality with an impatience and intolerance for those under him."
When scripting for the game, there should be options for different alignments. An evil character might choose to enslave, torture, and starve to death an enemy; while a good character might choose to politically neuter him and have him banished from the city.