Why do some guys have resentment towards attractive women?

They are insecure and probably had bad experiences with pretty people.

Some guys have a sense of entitlement like they are entitled to any woman they want so when they see someone that appears to be out of his league, they take their negative emotions out on her.

And there are some guys that are gay and are mad that they aren't her

It's all about the fear of rejection. Even just imagining, because a girl appears so unattainable, that she will reject you is enough to get some guys to say they dislike her without ever talking to her. They may actually be very interested in her but to appear like they are cool they say crummy things to try and demonstrate that they don't care.

It would definitely be someone who is insecure. Some of it does play into stereotyping, where it is assumed beautiful people are all in to themselves and look down on everyone. It isn't true, but it is a stereotype.

It's likely either the fear of rejection, actually being rejected by an attractive woman previously, or another reason entirely.

because they are afraid of them, they feel better about themselves when bashing beautiful girls because they know they can't get them. They find them mean because they always get rejected by them and they think they are shallow when in reality, they arent. I've had my friends talk about a girl in front of me and I asked them why, and they didn't have a straight answer. They just kept giving me stupid reasons, but not a good solid fact. They are insecure idiots who take their anger out on attractive girls because they can't get the girls...

I'll give you an example.

Bob - "Hey what's up, how are you?"

Jennie - "I'm sorry, but do I know you?"

Bob - "No, but I just thought I'd come over here and talk to you, it's much better than just staring at you because your beautiful"

Jennie - "Oh, well I'm sorry, but I don't really think you are my type, really sorry" gets up and walks away

Bob - Thinks in his head and says softly as she walks away, "what a bitch"...

Maybe they are gay?
Or have so much fear to talk with an attractive women, they have to get her to a lower level so he doesn't feel so bad.

But this looks like gay to me.