[quote=Sapphire]
Eris is showy, plus she wanted 'Less talking. More screaming.' not insta-death. I also think Eris began to become jealous of Maria and hoped to kidnap/kill her.
[/quote]

If Eris really wanted to kidnap/kill Maria she would have done so. In fact, if Eris really wanted to, she could steal the Book of Peace with Ease. And she's never shown having any trouble with the book being open. Personally, I think she let's the people have the book of peace, and just likes playing games with them. Then there's the possibility that she might be in love with Sinbad, which is also a valid theory.