Jenelle Evans didn't know who the hell she was dealing with in her road rage incident when she whipped out her gun, and she's lucky she didn't get her head blown off ... so says the other driver.
Robert Robinson Jr. -- the guy in the truck in the crazy video with Evans -- tells TMZ ... the "Teen Mom 2" star was the one who started acting crazy on the highway, so he admits he brake-checked her to try and chill her out.
He says once he saw Jenelle pull down his dirt road to follow him home in a rage, he alerted all his neighbors someone was coming -- neighbors who are like family to him ... and are all armed.
Robinson had a gun in his truck too, and tells us he only pulls it out to protect himself ... and shoots to kill. But he says he exercised restraint once he saw she had a kid in the SUV.
As we reported ... Evans didn't exercise restraint ... because she pulled her gun while her 8-year-old son sat beside her.