Daft.and inexplicable mistake

It was one of the highest rated tv shows in histiry,

Generally speaking, it is an established part of our cinematic culture that villains of all kind miss the opportunity of killing the hero immediately, once he has been overpowered. This is not only a standard element of narration in swashbucklers but in the adventure and action genre throughout. I just recall how often James Bond has been caught but managed to escape, because his opponents either were satisfied with him being imprisoned or they wanted to create an especially gruesome and fearful death for him but failed.
I think it mostly depends on the skills of the script writers, wether at that point we are able to relate to the villains decision or not. As for TAORH, I never thought, that Gisborn made an inexplicable mistake here, given that he is the governmental exponent of law and order and Robin Hood is the Outlaw. To have Robin sentenced in a court trial and afterwards executed in public emphasizes his authority: Common people would fear him furthermore, noble men would respect him, since he did not violate the law.