Manuela Ruda and her husband deny responsibility
"I got the order to sacrifice a human for Satan," Daniel said in a statement read out by his lawyer to the court in Bochum, near Dortmund.
Mrs Ruda said: "It was not murder. We are not murderers. It was the execution of an order.
"Satan ordered us to. We had to comply. It was not something bad. It simply had to be."
Defence lawyers argue that they are mentally disturbed.
Vampirism
Mrs Ruda told the court how she and her husband had lured their victim to their flat, and attacked him.
"Daniel struck him on the head twice with the hammer. But he suddenly stood up again and walked towards the television.
"Then my knife started to glow and I heard the command to stab him in the heart," she said.
After killing Mr Haagen, the couple cut an occult star on his stomach, drank his blood from a bowl and had sex in a coffin in which Mrs Ruda usually slept.
It had a scalpel protruding from his stomach, and a pentagram, the sign of the Devil, carved into his chest.
A prosecutor told the court he was horrified by the murder scene.
"I have never, ever seen such a picture of cruelty and depravity before. They simply had a lust for murder," he said.
Live burial
Mrs Ruda appeared at the court in gothic clothing, her head partly shaved to reveal an inverted crucifix and a target tattooed on her skull.
She said she had developed her taste for vampirism in the UK, where she spent time in London and Scotland.
"We drank blood together, from willing donors. I also slept on graves and even allowed myself to be buried in a grave to test the feeling," she said.
"I signed my soul to Satan two-and-a-half years ago."
The defence argues that they should be given lenient sentences, on the grounds of mental instability.
Several witnesses have testified that the couple suffered from personality disorders.
The court is due to reach a decision on 28 January.