That doesn't sound like particularly decent, responsible parenting, Nataleejah. The reason you think all these things are fine is because you were desensitised to the violence as a small child.
The Marvel comics were not written for children, they were written for adults. They have dark themes, violence and sex, and 'damagrd' people as heroes (and anti-heroes).
There's nothing wrong with that, btw, fantasy is healthy, and allows us to explore things that would not be acceptable I'm rl, but it just isn't appropriate for small children.
The PG/12A thing is difficult. In the past, things that past the censors and got PG would not now. But that's up to us to be responsible parents and not show children things they're not yet ready for. Grease, for example, had a whole storyline about Rizzo's pregnancy scare, but people seem to think it's a kids film. Back to the future is a good example. We bought the boxed sed cheaply, because we thought the children would enjoy it. We watched it first to vet it, and were shocked at the language deemed suitable (lots of crap, shit, can't think what else) and the attempted rape scene. I don't really want my 5yo walking round saying Holy Shit, it's just not okay.
And yes, my eldest (Y6) has friends that were taken to see Jurassic World and Spectre, and some were taken out crying, others were fine, but I know from seeing them myself that my DD is not ready for them.
Desensitising children to violence is harmful, and can lead then to make poor decisions or use unacceptable levels of violence themselves. Why would parents think it's a badge of honour that their child isn't scared or horrified by someone having their eyes put out, or by playing call of duty or whatever?