I'm worried about Gort
Klaatu: I'm worried about Gort. I'm afraid of what he might do if anything should happen to me.
Helen: Gort? But he's a robot. Without you, what could he do?
Klaatu: There's no limit to what he could do. He could destroy the Earth.
The scene on the baseball diamond/LZ where Gort is first noticed outside the craft, is one of the most chilling in film history.
Barnhardt: One thing, Mr. Klaatu: suppose this group should reject your proposals. What is the alternative?
Klaatu: I'm afraid there is no alternative. In such a case, the planet Earth would have to be... eliminated
Barnhardt: Such power exists?
Klaatu: I assure you, such power exists.