A 16-year-old boy accused of murder, alongside a 16-year-old girl and a boy, 15, said he thought he had done the right thing by attacking his alleged victim.
The boy previously admitted manslaughter but denies the murder of Alexander Cashford, 49, from Kent, who the three teenagers had accused of being a paedophile.
Earlier, the jury at Woolwich Crown Court was told Cashford had met the girl by chance, given her his phone number and exchanged about 75 messages.
The girl, who the 16-year-old boy said also knew about the plan to attack him, also denies murder, as does the 15-year-old boy.