At the end of the day Ros,it upset you,but I really don't believe that Errol suffered a lot.Sometimes,when an animal dies,there is twitching of their nervous system,which in our horror we think they are struggling and suffering,when they are not.
I'm basing this on seeing my 1st ex-batt(favourite)die.I can only describe it as reminding me of a fit,convulsions.I rushed to her and picked her up.
She died from being 'worn out',and maybe I can't describe it as peacefull,but the seconds it lasts, I honestly don't believe it stresses the birds.
I suspect you're playing it over and over in your mind,and making it seem more horrific to you than it really was.Take comfort from the fact you gave Errol a longer and happier life than a lot of cockerels get.Errol is at Rainbow Bridge,happy,scratching around with other chooks.He knows you always did the best for him,right up to the end.
I've dispatched rabbits,I can break a rabbits neck quickly,although it upsets me.I would wimp out dispatching one of my chooks,although taking it anywhere else for the final deed,i.e. a vets would be more stressfull for it.
You're being positive,treating this as a learning curve,and,your experience will help the rest of us.