I think a lot of people in the US agree with you, but I think you'd all be surprised to know there are a lot of juvenile drug dealers out there. Sometimes they're indies, others they're low on the totem pole of some gang. You can't solve the problem by killing these kids, also they want to be drug dealers because of the money and prestige, and the (perceived) control they get over their life.

There's also not a lot of evidence in favor of the death penalty as a deterrent. The most likely outcome of this policy is we just start executing hundreds of young men, mostly minorities.