I wonder if you are holding your self to an impossible standard. Would you judge others as harshly as you are judging yourself? The failure to thrive is not your fault, and neither is a mean bird. I mean there is a difference between being a horrible person who starves or beats their animals, and one that tries their best, and have animals that don't work out.
Believe me, we all make enough real mistakes, there is no point in feeling horrible for things out of our control. The way a cockerel is out of our control, to be honest, I think it is beyond his control. They have a very small brain and are largely hormonally driven. I know there are articles people post that blame the people keeping them, but a lot of experienced keepers remove problem birds and enjoy their flocks. It is a part of keeping chickens.
Mrs K