I think one of the factors is that he suddenly started to see Mokhwa as person and not a play thing/something to observe which can explain sudden shift.
He do look like someone wih (and here i do not remember diefinitions so pardon me if im wrong) either psychopat or sociopat tendencies and if im not mistaken they tend to abide by their own set of rules especialy if high functioning so Mokhwa getting switched from one category of things to another category of people can make change perplexing and feel sudden but believable enough
but this is only pondering opn another reason behind rapid attitude switch
Ilay gets better? I smh dropped it as translation was frustrating and i got lost in story plus interactions were getting bit frustrating too.