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."

http://www.wordreference.com/iten/sicuro

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.

what is wrong with my translation: Lui vuole tenere la gente sicura

I don't quite understand why al is needed before sicuro. Anyone?

I think that my translation means the same thing " Lui vuole mantenere la sicurezza della gente"

I think sicuro means safe as in not dangerous: "it's a safe neighbourhood". Al sicuro means safe as in not in danger: "you're safe in this neighbourhood".

why is it "securo"? I wrote "secura" because of "la gente" and was marked incorrect.

“Lui vuole tenere sicura la gente“ ?

Thanks everyone. Lots of good tips there