I don't completely get the total 180 from the grandpa
From what I understand:
He sees how much potential Youngguk has, but while as a grandpa he can see Youngguk going too far and hurting himself... as a director, he sees a relatively young actor really starting to shine, and even if it's hurting him, wants to push him even further
This moment where Youngguk looks "human" (i.e. breaks down because his mom is hospitalized) made grandpa take a step back and realize, hey, just because he's a monster actor doesn't mean he's a actually a monster, or a machine that can simply keep on going forever
At least that's how I read it