Who's talking about justification? I can look at the reasons why someone might do something that's bad or has negative consequences without believing that those reasons *justify* the behaviors.
Many people sell crack to feed themselves and their children, in the U.S. (where you get $100 or less a month for personal expenses and medical fees if you're on welfare). And, yes, to have the sorts of things other people get to have. They do it for the same reasons anyone from the middle class goes to work every day.
The violence usually comes in higher up at the top, where the people high on the supply side make deals worth tens and hundreds of thousands of dollars. A lot of people get killed for stepping in on someone else's territory (which costs the suppliers tons of money), or for not making good on a huge debt, or for snitching, or for any sort of disloyal behavior that threatens the others who rely upon drug dealing.
Gangs form around the mutual financial interest of those dealing within a certain "territory"--and then violence crops up around defending your territory from other gangs. Most importantly, in areas where gang activity rules the streets, gangs offer PROTECTION from the threat of violence at the hands of rival gangs.
Gangs are a lot like any other form of mafia.
