Seen this idea before with Green Peace poster that I've posted few days ago. This poster is done by a student Jan Drewniak in Toronto. It says "Gangsta 4 Life; means when you're in, there's no way out."
Gangs are about the sense of loyalty and pride between the gang members have for their sets. Dying for one's gang is a part of life, a necessary evil and an honor.
I think the ad should focus that there is a way out, to offer hope, that you'd better offer it for them who got troubled in such things. And there are some non-profit organisations that would like to help the gangster who wants a new fresh life to start.
Give them hope, before killing them alive.