Awwww, this is terrible news & just makes your heart ache! Sadly, even without accidents & medical issues, they won't ever live as long as we'd like them to.
I agree that there could have been a couple of things going on with her that contributed, but we (& you) can't know. (Did she have an underlying medical condition? Did she just freak out & have a heart attack? Did she suffocate? All of the above, none of the above?).
We DO know a couple of things for sure: 1) omg, this sucks!; 2) big hug to ya, you're feeling horrible & I understand; 3) you are SO going to do better going forward; 4) getting all of your future flock used to being handled will be a top priority (they won't all be cuddle bugs but will have to have regular inspections);
and 5) if she had mites, everyone has mites. Time to dust the remaining flock, coop & nest boxes.
I'm really REALLY sorry to hear that this happened to her & you. You are not a bad person & not a bad Chicken Mom.
As a tip, here are a couple of things that I do, which may be helpful.
I have a big, thick towel over my washer & everyone who's getting a thorough inspection stands there. If they need a soak in the chicken spa, I have a special, oval bucket that goes into the stationary tub (next to the washer). I am not above using treats as a bribe for being good & as a reward for letting me do my thing.
I used to have my husband help me with dusting by turning them upside down & holding them by the legs. Now, I have a squirt bottle (like you might see on a restaurant table, filled with ketchup...you know, round bottle, pointy tip with a cap?) I fill that with my mite dusting powder & when squirting it I can get their backs, bums, under their wings, necks, chests, feathered egg pants...everywhere! It seems less stressful for them & it's definitely less stressful for me!