As with any career or job there will be troublemakers that make the whole look bad. What happened at Abu Ghraib will never happen again as those who were involved were court martialed and punished. They are now serving time in military prisons here in the United States. They will be dishonorably discharged upon their release and will have a felony on their civilian record to show for it.
You are correct it is about the mentality, but the whole point of basic training is to 'get in your face' and see what your made of. They will break him down, but at the same time they will build him back up into a confident, well-directed person. I'm not saying the military is not a cure-all, but for many people it does help when they are running out of options.
The reason why I am saying the military is not a cure-all? My jackass exhusband was ex-military. He still had a record long as my arm after everything came to light. However it was said by his own father that when he was in the army, he was actually doing really good because he was right there where someone was watching him, giving him direct orders, and he was receiving high marks from his commander. His father wished he had stayed in the military, he would have done extremely well. After he got out, that's when everything went down hill. The military can be good for those that need this type of hardcore direction in their life with the comraderie of their unit. Your brother is low on options and to him the military may have seemed like the lesser of two evils. Encourage him on this endeavor, instead of worrying. I understand it's your 'motherly instinct' kicking in, which tells me you will be a very good mother one of these days if you aren't already!