Anthony Bird, 50, died in hospital after the alleged prolonged and ferocious attack in Victoria Park, in Tipton, on July 26 last year.
Defendants Steven Bennett, 38, and Suni Gill, 33, had been visiting the park with a female friend when they confronted Mr Bird accusing him of being a paedophile, an unsubstantiated claim.
Giving evidence from the witness box Steven Bennett said the woman told him that Mr Bird had taken photos of her children.
He said when he approached Mr Bird to ask what was happening, the older man swore at him.
Bennett said: "He headbutted me out of nowhere. I was shocked. It landed between my chin and my lip. At this point I didn't know what else was coming. I went into defence mode.
"I threw a punch at him in self-defence. It caught him on his right hand side.
"I'm not sure of the force used. He was bleeding. He was holding his mouth with his hand.
"He turned to walk off. I turned away and he spat toward me. I don't think any spit landed on me. It happened as he was walking away."
He told the jury that Gill then approached due to the commotion and followed Mr Bird.
"Suni had been drinking heavily. Mr Bird was making a racial comment I thought that was because he was Asian. Obviously I thought that was why Suni was chasing him. Mr Bird tripped over. When I got to them, they were already fighting," Bennett added.
The jury previously heard that the defendants rained kicks and punches on the older man as he lay defenceless on the ground.
Mr Bird died in hospital 17 days later.
Bennett, of Bevan Road, and Gill, of Shore Road, both in Tipton, both deny murder. The trial continues.