This is the 18890th most frequent Korean word.
심할
"Too severe" or "excessive."
In this sentence, "심할" refers to the severity or intensity of his reaction.
그의 반응이 심할 만큼 과격했다.
His reaction was severe enough to be extreme.
Here, "심할" is used to describe the intensity of the cold weather.
날씨가 오늘 참 심할 정도로 추워요.
The weather is extremely cold today.
"심할" here denotes the excessive or intense degree to which the movie induces fear.