John Hunt said Thomas Orchard, who had schizophrenia, was "trying to bite and spit" at officers during his arrest in Exeter city centre, as he verbally abused passers-by.
Mr Orchard's mother Alison previously told the court he was never aggressive.
Three officers deny manslaughter charges.
Custody Sgt Jan Kingshott, 44, and civilian detention officers Simon Tansley, 38, and Michael Marsden, 55, all deny two charges.
The first charge alleges the trio carried out an act or series of acts which unlawfully killed Mr Orchard and the second alleges they unlawfully killed Mr Orchard by gross negligence.
Mr Orchard, a church warden, died in hospital in October 2012, seven days after he had been arrested on suspicion of a public order offence by Devon and Cornwall Police.
Bristol Crown Court had heard how Mr Orchard was held down for more than 20 minutes by police officers and detention staff at Heavitree Road police station in Exeter, before he stopped breathing.
Mr Hunt said the police handcuffed Mr Orchard and "put leg ties on because he was shouting out and trying to bite and spit".
"He was shouting, 'I know what you look like, I will find you' and he was going to kill them," Mr Hunt told the court.
Mr Hunt, who was shown CCTV footage of Mr Orchard's arrest, agreed the officers used "reasonable" force.
The trial continues.
A man who died after being restrained by police was threatening to "kill" officers during his arrest, a shop owner has told a court.