In my stories, Guardian Angels are disillusioned, angry, jealous and homicidal entities who have grown to resent humans and are tired of their "jobs".
They are sick of listening to desperate prayers and choose to get rid of whining humans instead of helping them. Think of an entire customer service complaint department that goes ballistic because of nonstop complaints.
Ever get so tired of someone coming to you constantly, asking for help, that to them you become some sort of savior/therapist? There are so many people who are praying to them, expecting them to rescue them from their troubles. So too many prayers can become overwhelming and with too many people crying out for help, guardian angels in my stories take matters into their own hands.
They decide to do some mercy killing. They target those people in despair and end their lives for them. They can appear in human form or enter your mind and act helpful, sympathetic and charming to get into your head.
And they are Vampires. So not only do they get rid of you, they make a meal out of you too.
So be careful who you pray to!