I suppose since shadow Beru is intelligent enough to talk, he might be intelligent enough to betray as well. He swore to eternally serve Jinwoo, but if you have intelligence and can talk, you can say anything without meaning a word of it. Though that being said, considering how mighty Jinwoo's necromancy skill is, it would seem like a strange oversight. His UI did recognise shadow Beru's power level as well, so it's not like he would have hit an integer overflow and thus the result would be unpredictable. But then again, Jinwoo simply got the necromancy skill as is, so he doesn't know anything about it, aside from how to use it.