London is in the grip of a spate of violent crimes which have shaken the public’s confidence in the capital’s police and politicians. Their faith in the criminal justice system is hardly going to be restored by the arrest of a 78-year-old pensioner on suspicion of murder after an intruder was killed in his home.

Certainly the police need to investigate the circumstances of the death. From what we know, two burglars entered the house in the dead of night, one armed with a screwdriver. In the ensuing struggle, one of the intruders was stabbed and the other fled. The pensioner suffered bruising.

So why is it necessary to detain the householder on suspicion of murder when he was evidently defending...