I'll just add this. Many Christians seem to praise God when things go their way but then choose to ignore all the shit that goes bad that he doesn't seem to care about and it annoys me to no end that they somehow think they're special enough to get all the good things they want while some kid suffers through a horrific war, or being sexually abused or whatever and God doesn't do shit all about fixing the problem. The kid has no free will, it's a kid.
So, yes, I understand why they think God is a genocidal narcissist who would turn a blind eye to slavery when it suits him and why Christians would happily support slavery. Hell, Catholics and Christians can't even seem to get off their ass to stop sexual offenders from continuously gaining places of power in their organizations.
Free will has little to do with your success or failure at times, and quite often humans are sucked into shit they have no control over, so saying "Well we have free will so that excuses God from not stepping up to stop evil" seems like a pretty big cop out.