Jenelle Evans keeping a loaded gun in her car -- in reach of her children -- proves she's not using sound judgment as a parent and doesn't deserve to have custody ... so says her ex and 2nd baby daddy.

We spoke with Nathan Griffith -- who shares 4-year-old Kaiser with Jenelle -- and he tells us he's incredibly concerned for his kid's safety in light of Jenelle's recently aired road rage incident on "Teen Mom 2."

He believes the video shows her kids are being raised in an unstable and hostile environment ... so his fight to get custody of Kaiser is well justified.

MTV

There's also this ... Nathan actually sides with Jenelle's husband, David Eason, who berated her for following someone home and confronting them. Nathan says it could've turned into a bloodbath, which is true ... 'cause the other driver told us he was armed.