Thank you for the photos.
So you only have 5 hens including Big Red + the meat bird correct? Yes so 6 total.
What are your plans for the meat bird? Is she able to go outside at all or do her leg troubles cause problems? We are go to keep little, out meat bird. She goes outside all the time and walks around. She just walks with a limp.
You mention that you found the cockerel (boy meat bird) in the corner of the big coop, but that is not normally where he "lives" correct? Correct, they do not stay in that particular coop, but they (the meat birds) go in there when the other chickens aren't in there. Your temps of 102+ I would consider that as a possible contributor to his death. If he were being chased in the coop/outside, stress from being just a meat bird plus heat could be detrimental. Yes, that is true.
In the photos all your birds look to be panting/under stress - I assume due to heat. How much ventilation do you have in the big coop? Both coops have great ventilation and are equipped with fans to circulate the air.
I'm sorry for you loss, but I personally don't think that Big Red "killed" the cockerel, she may have chased/harassed him like established flock members do when they are not happy with new additions.
Unless you actually saw the killing, I would not be blaming her. As for what happened 1.5yrs ago, unless you saw what happened there, again, it would be hard to know. I too have had a bird die in the run (I saw it) one or two birds were standing over the body when I walked outside to get the body and examine her - if I had just walked upon the "scene" it's possible I could have thought they killed her, but I would have had to see evidence. I won't be killing Big Red because of the incident. She is good around the flock she has now, which took some doing, it is just with new introductions. Unfortunately, were not around with both incidents, but we have experienced her with other chickens and she is relentless if one tries to challenge her.
Also, the first time our Barred Rock was killed, the reaction of all the hens when big red was reintroduced to the flock after about 3 weeks of solitude (they stayed clear and it was very obvious all of them were leary of being around her. Thanks for all of the comments and I appreciate the feedback.
Just my 2¢