Boxing champ Deontay Wilder tells TMZ Sports ... the only reason he agreed to fight Internet troll Charlie Zelenoff was to teach him a lesson about bullying and SAVE HIS LIFE.
Wilder -- the reigning WBC Continental Americas Heavyweight -- says Zelenoff has been harassing him for 3 years ... calling him the N-word in texts and in phone calls and even threatening to harm his young daughter.
"He said he was going to murder me. He said he was going to tape my daughter's mouth ... when dealing with my kids, that's below the belt. That's what really got me."
