Why Albus Dumbledore die easily?

Answer 1

Many things become clearer if you read the last book, Harry Potter 7 (Harry Potter and the Deathly Hallows).

Spoiler warning: You may want to read the book instead of reading on - or wait for the movie.

Superficially, it seems that Dumbledore was weakened by the nasty drink he had, prepared by Voldemort. That's what Harry thought, but this wasn't the main reason.

Remember that in book 6, it is mentioned several times that Dumbledore's hand was damaged? In the movie it was also mentioned briefly. It turns out that he imprudently put the ring on, forgetting that as a horcrux, it was probably cursed. The curse, according to Snape, was unstoppable; Snape could slow the effect, but Dumbledore only had about a year to live. So, Dumbledore told Snape that he should kill him. Among other things, this made it easier for Voldemort to trust Snape. Snape didn't like this assignment, but eventually agreed. When Dumbledore said, right before his death, "Severus, please!", he was not pleading for his life... quite the contrary.