I can tell you with 100% confidence, if there was a volunteer petition to sign, there would be a line out the door. Believe me on that one, there is absolutely no doubt. The world has hated America for a long time and that's by no means the fault of any one single party. It's because in general, America makes everyone else's battles their problem and doesn't know how to keep it's nose out of anything. For instance, who the hell cares how any other country treats their people? We have so many issues here, in so much financial debt, yet we keep aiding other countries....... Lets fix our own before we stick our nose into other peoples issues. Nigeria is a shithole, yes it is, arab countries treat their people light shit, yep, so what, is it our problem, no it's not. Never was and never should be, move on, worry about America first for once.
While I agree with your general premise and what your ultimate goal is, your theory has a couple flaws:
1) if an army of border-guarding murderous volunteers are so willing... Then why isn't anyone at the border right now? What's stopping them? The fact that it's illegal I suppose? I'm actually asking why people like you whom you claim are a-plenty and would gladly love to go stand at the border for days on end for FREE (volunteer right?) are not actually going there as we speak. Probably because it's a little unrealistic.
2) lets play the numbers: 2,000 miles of border at 5,280 feet per mile = 10,560,000 feet of border. How many feet between each person in this wall of volunteers? 100? ok so that's 105,600 people who need to be standing guard 24 hours a day, in the desert for free, with a lethal force license. Oh wait, 24 hours isn't realistic right? Ok so let's say 8 hour shifts. That's now 316,000 people. I don't know about you but I don't have 8 hours to spare. Much less 8 UNPAID hours to spare every day. Realistically, maybe 2 hours a day huh? Ok so now we're at 1,200,000 people if we all take 2 hour shifts every day, 24 hours a day 7 days a week.
3) let's say there actually is 300,000-1,000,000 people willing to stand guard in the desert for free 24/7/365. Who decides to let these folks have a license to kill? Who does the vetting process and weeds out felons or undercover cartel agents or people who can be bribed? That process takes time and money. Remember you're suggesting we allow random humans to stand in the desert and shoot anyone that tries to run pass them with lethal force, for free.
4) alright, so we've now found this 1,200,000 volunteers who are sane enough to know right from wrong yadda yadda and they are expected to leave their homes from let's say a random place like Maryland and now afford to live on a zero dollar salary in the desert? Although ironically by doing this, Trumps wall would create 1,200,000 jobs from the volunteers leaving their current job to volunteer for murder. So there's that positive spin.
It's just unrealistic. Sounds good on paper and overall your intentions are good for what your goal is, but the road to hell is paved with good intentions.