Avangers infinity war

SPOILERS AHEAD:

The Avengers are dead because Thanos snapped his fingers and this snapping of fingers indicate the use all the infinity stones to destroy half the population of the universe. And in this half the population, some Avengers were included along with other protagonists.
Thanos wanted to destroy half the population because he feels that it was his destiny to bring balance to the universe by not exhausting it's resources, etc.