The Volturi wants to kill the Cullens but is hesitant because of Bella's shield, Alice sight, and their connection to the Shape Shifters who they know nothing about. So the Volturi decided to capture a wolf.
That was Aro's biggest mistake.
This is mainly a love story but it's very dark. It takes place in a setting were most of the characters are insane, goes insane, or gain their sanity. Its not a story filled with flowers and cotton candy (although there will most definitely be flowers and cotton candy) it's not always sunny in Volterra.
READ AT YOUR OWN RISK.