Why did Quint scream like a little girl when he died?

Let's have a huge predator eat your legs while you're alive and conscious, and we'll see how much dignity you can maintain.

Because it hurt?

Dramatic effect. If he had done as you suggest, and as we all expected, it would be a somewhat anti-climactic scene.

A terrifying agonizing death.......yeah I'd be shrieking in a high pitched voice too

Is there a dignified way to get eaten by a shark?

He screamed like that because, if you look closely while the scene is playing out, you’ll noticed that in the center of the screen, the biggest great white shark in history is biting him to death.

I have a feeling being bitten in the stomach by a Great White Shark doesn’t feel particularly good.

You have this 3 ton 25 foot Great White not only chomping his legs, but then chomped right down on him forcing him to spit up blood before dying.. I'd probably scream like a girl too, just like the guy who screamed like a girl who lost his leg in the john boat