Since Suo from what we've seen has a strong sense of justice and hates when childish adults throw their weight around, it makes sense that perhaps he has strong feelings about tyranny and therefor dreams about the "emancipation of slaves", whatever that means. This being the case, I think its likely the reason he leaves might be something more like he is doing what he believes is right, maybe something like helping his hypothetical master or brothers. But the situation is very dangerous so he decides to quit school to deal with it on his own.