Not everyone can read novels, drama and webtoons are much easier to watch/read and they are also more discriptive than novel, this is the whole point of an adaptation, to increase the range of viewers but the now a days there's a trend of adding unnecessary and sometimes unfavourable and unpleasant plot twists just so to keep the people who have already read the novel hooked.
In the drama, their are many small deviations from the novel and a major one too. I personally would have preferred if the drama went the same as novel, small deviations would have been appreciated but the last one was just too much.
Please read the novel if you can, because many will ultimately want read the novel because of drama being deviated from the original novel, specially at one specific point and then the drama will act as nothing more than a spoiler not letting you enjoy the novel as you could have.