A man's ethical behavior should be based effectually on sympathy, education, and social ties; no religious basis is necessary. Man would indeed be in a poor way if he had to be restrained by fear of punishment and hope of reward after death.
It's not a rock problem either
Here's a guns + scripture billboard from somewhere in the U.S. of A.:
Cain killed Abel with a rock
It's a HEART problem, not a gun problem.
Okay. So Cain killed Abel with a rock.
But in December 2012, Adam Lanza killed 26 people at a school, including 20 kids, with an assault rifle.
Over 4,000 more people in the US have been killed with guns in the 6 months since then.
Over 32,000 people in the US were killed with guns in 2011 alone. Subtracting over 19,000 suicides by gun, over 1,000 accidental or undetermined gun-related deaths, and a scant 600+ "justifiable" homicides by gun, around 11,000 gun deaths in the US in 2011 remain classified as murder.
Every year about 30,000 people, 10 times the death toll from 9/11, are killed with guns in the US. Counting only murders as the problem addressed by the billboard, that's still 11,000 people, or about 3 times the death toll from 9/11, murdered by gun every year in the US.
You're wrong, billboard. It's a gun problem.
(The false equivalency on the billboard is the same as that in this cartoon: pretending that the gun problem in the US is one of "a guy killed a guy." At 11,000 murders a year, the problem in the US is not "a guy killed a guy.")