Andrew Molloy, 25, died after being punched by former boxer Louis Veitch, 22, who moments earlier told onlookers: "You might think I'm a short fat muppet but I've got a killer punch."
The attack was caught on a graphic CCTV clip, re-released by Greater Manchester Police (GMP) to launch their campaign.
Mother-of-three Jackie Molloy, 48, said: "It shows just how easy it is. Just one punch and that's it, his life was ended, somebody's life is over in a second and a family are devastated.
"My son is dead because somebody probably had a bit too much to drink and just thought he would be clever. The minute you decide to punch somebody that's it, it can be life or death, that one punch can be life or death to somebody.
"The message is, by all means go on and enjoy yourself with your friends, you're all only young, but if you're in a pub and someone is being a bit aggressive, get out as fast as you can."
Mr Molloy had intervened to stop trouble involving Veitch in Oldham on March 15. He was knocked off his feet by the punch. Veitch, from Blackpool, was jailed for six and a half years.