The Bird is angry because the pigs stole there eggs.
It strikes back by slingshoting himself to kill the pigs.
That is why it is called "Angry Birds".
In the Angry Birds series, the Birds are the main protagonists of the Angry Birds series, which come in a variety of different species. Their main goal is to retrieve the un-hatched eggs that their arch-nemesis, the Pigs, stole. Since then the birds care more about revenge than anything, and hurl themselves at the various structures the Pigs constructed. There are 16 bird species, including the Mighty Eagle.