Also, it isn't really in the code.
They are allow to kill defeated opponents, they prefer not to as the rather allow other people to bring people to justice.
I don't know why they think this was more than Obi-wan not able to bring himself to kill Anakin.
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Grand Inquisitor and 5th Brother (the Green Dude) both die during Rebels.
And the other Inquisitor in the series is the 4th Sister and isn't a Cathar.