Tyrann Mathieu -- I Know Will Smith Murder Suspect ... 'Hating Ass Coward'

NFL superstar Tyrann Mathieu says he knows the man suspected of shooting former Saints defensive end Will Smith ... and says the guy has always been a "hating ass coward."

Turns out, Mathieu -- aka the Honey Badger -- and the alleged shooter, Cardell Hayes, were both high school football stars in New Orleans ... though Hayes is 5 years older than Tyrann.

Still, Mathieu went to Twitter and wrote, "I know that dude, he been a hating ass coward.... Never knew he'd grow up to be a killer tho.... May you get what you deserve coward."

But there's more ...

Mathieu went on to blast rap music ... claiming it's "brainwashing [people like Hayes] to believe drugs and killing are the cool thing to do."

He added, "That sh*t ain't cool, cool is being a father and being able to provide ... cool is handling things like men not like cowards."

Tyrann also said he'll be dropping a rap album of his own to combat the negativity.