The ex-NBA star comes from a tough background and knows how bad things can get when you push a real-life gangbanger too far.

Barnes insinuates that XXX got in too deep with real killers and learned the hard way what happens when you mess with the wrong people.

Then comes the warning to Tekashi -- who Barnes says is "running in every hood, talking sh*t to everybody."

"Bro must have a death wish, man, cause these motherf**kers are not playing. They eat for a living. They eat motherf**kers like this for a living."

The main message from Barnes ... the young rappers need to learn that gangs and gun violence only leads to one ending -- death.