"He wants to keep the people safe."
Translation:Lui vuole tenere la gente al sicuro.
I think I figured this one out. By itself, sicuro is either an adjective or a noun. As an adjective it means "safe" and as a noun it means "a safe place."
Used as an adjective, we could get "He wants to keep the safe people" (those without guns, maybe) but to get "He wants to keep the people safe" we need an adverb phrase, which we can get via a preposition. So Lui vuole tenere la gente al sicuro literally means "he wants to keep the people in a safe place."
As always, some feedback from a native speaker would be very welcome.