That's not the situation that's being discussed, though.
It's more the Jean Charles de Menezes situation, where he was shot dead for jumping a barrier/running from unidentified, plain clothed, armed men.
I can empathise with the police thinking he was a terrorist suspect and was reaching for a trigger, but they 100% shot to kill (seven times in the head), and an innocent man died.
It was an impossible situation for the officers to be put in, and I do back them in that case, but all Corbyn et al. is saying is that these people SHOULD face a trial, in theory.
Corbyn did say, in the same interview, that Jihadi John deserved to die (or words to that effect) - it's just that EVERYONE, no matter what level of crime should face trial.