The frightening scene in this movie is the one where the Monster has been playing with little Maria, looks down at the flowers, then at her, and with a childlike smile on his face grabs her and...
There's also the scene where Frankenstein and his mentor discover what the Monster did when it finally got its hands on that jerkass Fritz.
On the subject of Fritz, his treatment of the Monster qualifies this as well. Considering that its intelligence appears to be on roughly the same level as a young child's, the way Fritz torments it comes across as incredibly cruel.
The monster sneaking up on Elizabeth and cornering her while she's alone. While the lustful growl he makes is one of the movie's funnier moments, it still makes clear that his intentions in this scene aren't as innocent as they were when he caused Maria's death.
