Before this blows up into a 'Who's better' argument - it seems to me the study just shows how limited society's perceptions are. It doesn't matter, of course, whether someone is Atheist, Agnostic, deeply religious, or whatever. And it doesn't matter the religion, Christian or non-. The study created a hypothetical scenario. Whatever.
When one person does horrible things, there are too many reasons why to even begin to list. Hypothetically? Someone who tortured and maimed small animals as a child grows up killing a whole lot more than 5 homeless folks. And it has nothing to do with his (or her) religion and everything to do with them being a sociopath.
Looking back through history, large groups who have assaulted, tortured, killed, etc. are professed followers of someone...more often though, that someone is God. They're the ones that suck, as far as I'm concerned for creating an automatic association for many others that God equates to war, terrorism and retaliation. And when once person is responsible for doing the same, or getting others to do it with/for them, it is self-professed to be 'God's work'.
Just for the record, I am a devout Christian. And I have had, and still have, and foresee to keep having friends and associations in my life who are atheist or agnostic that I like a heapin' helluva lot better than some fellow Christians I also know.