I think he learned to shoot before he was a Marine.
Good marksman can be trained but becoming a superb marksman is a lifelong pursuit.
The training is very stringent and possibly the best in the world in terms of producing excellent marksmen. A person having had that type of training can utilize the basics and shoot well even with somewhat inferior equipment.
As far as putting my service on the "side of the angels", the thought hadn't really entered my mind. For that matter, do you remember Charlie Whitman? He was the guy in the tower on the Texas campus that killed 14 people and wounded a few dozen more. Yep, he was a former Marine.
Not all Marines are "angels". However, it is relatively safe to say that most all Marines are good marksmen.
No argument from me.
But Oswald failed at everything else he tried. I don't remember his USMC record but he was certainly not a stand out and was depressed enough to head for the USSR soon after.
And I had forgotten about Whitman. Doubt a WHOLE lot that either acted as they did because they'd been Marines, more likely joined trying to fit their self-image.
I think he learned to shoot before he was a Marine.
Good marksman can be trained but becoming a superb marksman is a lifelong pursuit.