If you are willing to dedicate yourself, you can certainly learn to be a real badass without joining the military.
First, go enroll in an offensive martial art that uses realistic fighting skills. Make sure it’s one that does full-contact training. That means no Tae-Kwan-Do or Tai Chi. Krav Maga, Brazilian Jiu Jitsu, Thai Boxing, that sort of thing. This is as much to teach you how to take punishment than to teach the skills. It’s one thing to have good martial arts skills - it’s another to be able to employ them AFTER you’ve already taken a good punch in the face.
Now, go take some defensive driving courses, and you can even sign up for one of those corporate anti-terrorism schools, where they teach you evasive driving techniques. There are also a number of anti-terrorism schools that will teach you how to shoot guns and protect yourself.
If you want to take your driving skills to the next level, sign up for some courses in rally racing and road racing. Learn how to push cars to their limits.
Lockpicking, boobytrapping, and other dangerous arts can be learned from books, if you’re willing to practice. The Paladin Press has tons of books on this stuff.
For handling guns, go to a range and take every course they’ve got. Enroll in some IPSC pistol courses, which are as close to combat shooting you can get while still engaging in a legitimate hobby. Then get yourself to the range several times a week, and practice with focus. I’m always amazed at the number of people who show up at the range who’s idea of practice is to just roll a target downrange, pick up their gun, and blast away as fast as they can. You need to practice hard. Take a few shots, examine your stance, think about what’s going wrong, correct it, shoot again… It would really help if you could find a very good instructor.
Then you’ll want some fieldcraft. A lot of this can be learned from books, if you are willing to apply it. I did this once - studied how to live off the land, make cooking implements, recognize edible plants, etc. Then I tested myself by hiking out into the bush for a weekend with nothing but a knife. It was fun, in a masochistic kind of way.
Learn to hunt. With a camera, if you don’t like shooting things. But many of the skills hunters need (silent movement, concealment, etc) work well against people, too.
Keep it up, and in a few years you can be a walking killing machine. However…
What makes an American soldier so effective is not his own personal badass-ness, but the incredibly effective tactics the U.S. military has built up. So if you took a hundred guys who are individually badasses, and put them up against a couple of squads of American soldiers, none of whom were anywhere nearly as tough, the soldiers would still kick your ass. That’s the problem terrorist groups like al-Qaida have - they may be tough guys individually, but they don’t have the tactics nor the tactical support from the air and ground that the Americans have.
One other caveat - like all skills, you have to keep it up for it to be effective. I’ve got a black belt in Karate and I used to have tournament-level shooting skills. Now I’m out of shape, out of practice, and the only person I’m a threat to is myself. My idea of roughing it now is to go camping without an air mattress. Being a badass is a lifelong commitment. It’s also a state of mind - I’ve known real badasses, and even at my physical peak I wasn’t one of them. I’m just too nice, and too soft. Military training helps work the softness out of you. Without that, it’s really very difficult to make yourself tough.
As they say, General George S. Patton was a great man. Dick Van Patten is a nice guy. But you wouldn’t send him to whip the Nazis.