Equiano, Slave ships/ middle passage

I don't understand how people could be so horrible. How could anyone watch or make some one suffer through that. Even in the view of the trade/economy, they lost profit due to the condition they forced. I don't get how people could be so evil and more importantly, where did that evil go? Human nature does not end, do we hide it better now or do we justify it as they did? Do they feel and think as if its not big deal , maybe similar to a cause such as abortion. Packed ships where people would die from the smell, how could a person force any one to endure this...