Wait I don't understand, why is he suddenly act like that?

maybe the spell that the assistant shadow guy used to separate them from the MC’s consciousness? he did say “well the ML won’t go insane right?” maybe it’s a side effect from casting it to fast

Growe had to link his consciousness with Enid's because Enid wasn't waking up after casting the spell to make Growe an adult again. While they both were unconscious, Seth (the gray haired mage) suddenly severed the connection in order to to wake Growe up to deal with the attackers. Growe's feelings for Enid accidentally got left behind in Enid's half of the consciousness world. Growe's personality had changed a lot after he met Enid, he learned humanity and empathy because he developed feelings for Enid. Without those feelings, he goes back to being a psychopath