It's probably best to think of "sicuro" as a noun here. (It doesn't change for gender and number.) Something like "keep the people in a safe place."
I think it is only because it says 'al sicuro' not just sicuro which changes the meaning slightly
Interesting. Are there any other adjectives that you do this with?
The only thing that comes to mind is the word "parte", which has different meanings (some fairly adjectival) with different prepositions. http://www.wordreference.com/iten/parte
What does 'al' do here?
Apparently "al securo" is something like "in safety."
can this done without the "al" though?
He's the hero we need, but not the hero we deserve...
Im the batman
Può essere "lui vuole tenere la gente sicura", certo? È la stessa cosa, non è?!
Secondo http://www.treccani.it/vocabolario/sicuro/ , sì, ma "al sicuro" sembra essere più comune.
"He wants to keep the people in safety" is wrong?
Sorry but I can't give a clear explanation; I am an American English speaker.
That's just not how we would say it in English.
We keep things "safe", not "in safety".
Sometimes, we keep things "in a safe place".
Why not "He wants to keep people safe"?, without "the" (the people)? Well, my sentence was marked wrong!! Not including "the" does not change the meaning!