Actually if you look at it closely, he has always been polite to everyone that talked to him. The only time he's being a jerk is when he first met Hwagok and he's too hungry to care etc and him being a jerk afterwards might probably be because he's emotionally constipated after so many years of being alone, and found a plaything. Once he realized he love Hwagok, he mellowed down to how he supposed to be given the chance that he could stop being alone...
There are a few scenes that's weird even for me, I agree, but overall, he's just a lonely guy that found love (๑•ㅂ•)و✧
That he is.
I feel like Lee Bin's personality shift was too drastic. Not going to say I don't like him but I thought it was too extreme too fast from the type we see at the beginning.