I think it was very vaguely mentioned that Ein didn't run away. He's a slave, so running away would mean being hunted down and punished. I think it was mentioned (or implied) that the father of his previous master sold him, because that guy was absolutely obsessed with Ein to like, a really extreme degree. The father sold Ein to separate them, probably in hopes that his son's obsession would go away once Ein was gone.
My mind is going places to how gross it could have been for Ein to run away and stay in this new abusive place rather than go back... Ein was a child and his father was obsessed with him... (⊙…⊙ )RUN FOR YOUR LIFE EIN! Σ(っ°Д °;)っ
Wait I think I might have phrased my response a bit weird. Ein's father wasn't the one obsessed with him (not sure we even know who his parents were).
Ein's previous master was some lord. The lord's son was the one obsessed with Ein.
Base on his reaction about having a room for his, Ein's previous master is much more worse.
Eitherway, the son could be a grown as adult and Ein was a smol child ╥﹏╥
I can't understand one thing... Isn't Ein run away from his previous master, because of sexual harassment? So, he run away from one perpetrator to second, and he was okay with this new conditions? However, I understand he have better "money" conditions in here, but when I think about it now it have less and less sense