A world in chaos

In my story, everybody's life energy is "leaking out" or something like that, meaning their days are numbered. But they all have devices that can absorb the life energy of any recently dead people nearby. I know that people'd start turning on each other, but I can't imagine the specifics. I mean, can people still band together and make gangs, or everyone will just fight alone? Can some people kind of make a safe haven, with guards and all?

I don't see any reason why people wouldn't make gangs. You'd have strength in numbers, loyalty to one another, and a group as dedicated to gathering the dead as making the lives of others miserable. There'd likely be some hierarchical issues, and certain bonds could get broken in tough times, but then again those are the kinds of things that might make for a compelling story.

As for the question of a safe haven...well, I suppose it's possible. But I'm assuming that just because there's a safe haven doesn't mean that there'll be a crime-free utopia, especially if lives are on the line (in more ways than one). It wouldn't take much for one man to crack under pressure, kill someone, and absorb life energy; a violent act like that could cause a domino effect leading to much bigger problems — riots, questioning authority and its effectiveness, all sorts of things.

So generally speaking, I think you can play your idea in a number of ways. It's not so much a matter of "could this work" as "what can I do with this". Just give yourself some time to think over your story elements and the reactions/results that would occur, and I think you'll be closer to the product you want.

Hope that helps you out a bit.