It's more that their options are severely limited, and the military offers a paying job that could lead to a career, or at least pay for college, which could lead to a career. Getting the Hell out of bad living situation, either a terrible home town, or a rough family situation, is another good reason. Foster kids age out of the system at 18, and often go homeless. The military looks like a positive viable option from that perspective.

There are a LOT of reasons that people join the military, and VERY few join the military to kill.