Ross Parker, 17, of Bozeat Way in Peterborough, was attacked with a hammer and fatally stabbed in September 2001.
Shaied Nazir, of Cromwell Road in Peterborough, admitted kicking the teenager as he lay on the ground, and trying to spray him with CS gas.
Mr Nazir, 21, Ziaraff Mahrad, 21, Ahmed Ali Awan, 22, and Sarfraz Ali, 25, all deny murder.
Mr Nazir, the first of the four to give evidence, told Northampton Crown Court he had seen Ross Parker being kicked as he lay on the ground, and had kicked him a couple of times himself.
He said he then saw Sarfraz Ali hit Ross with a hammer, and that had prompted him to walk away.
After the attack, he said he and his co-defendants went back to his shed, where he realised their trousers and boots were bloodied.
He said he saw Ahmed Ali Awan holding the bloodied murder weapon, and had asked him if he had stabbed or only slashed someone.
He said Mr Awan had replied he had "slashed him", and had indicated the left side of his chest.
Mr Nazir said he had wanted to believe his friend, but could not, as there had been too much blood on the knife.
Mr Nazir has already admitted washing the foot-long knife after it had been used to kill Ross.
The case continues.