Why does everyone want Bella's blood in twilight?

No one would like to kill Reneesmee, Edward and Bella's daughter except for Irina, a member of the Denali Coven. Irina reported to The Volturi that the Cullens created an immortal child, and should be killed. The Volturi came to the Cullens, and inspected, but then found out that Reneesmee was only half-vampire, or half-immortal. Then, sadly, Irina was killed for reporting false information to The Volturi.