A teenager wrote about "the sweet horrors of blood" just days before killing her parents.

The day after Ashlee Martinson’s seventeenth birthday, she killed both her parents.

Her stepfather, Thomas Ayers, was shot in the head twice. Her mother, Jennifer Ayers, was stabbed multiple times.

In the weeks leading up to the crime, Martinson penned disturbing poems for her blog, Nightmare.

In one poem, Martinson describes herself as a “psychopath in the dark, marvelling at the sweet horrors of blood that I thirst for.”

“I clean the dry blood off my tools from a previous session,” the poem goes on.

“The last body has been disposed of just hours before, yet I have not been satisfied with the pain, agony and blood.”

Martinson’s story is one of a deeply troubled young girl: abused by her mother’s partner for years, ignored by friends and family.

In interviews, Martinson alleges that her stepfather regularly beat her mother, herself and her three younger sisters, once telling the four girls that he had “shot their puppy and fed it to the bears.”

Avery would wake Martinson up at 4am with a list of chores to be completed before the end of the day, confiscated the money she earned at her part-time job and only permitted her to leave the house to socialise once a week.

Martinson told her court her stepfather would often call her an “ignorant bitch” and shout at her that she would amount to nothing in life.

He would often hold a gun to her mother’s head as a threat.

On the day of the murder, Martinson heard Avery assaulting her mother. She ran away from her family home in Wisconsin, intending to stay with her boyfriend, but her stepfather found her and brought her home.

Martinson picked up a loaded shotgun, intending to kill herself, but at the last moment turned the gun on Avery instead.

She shot him twice, to “ensure that he was dead”. She believed she had done the right thing.

But Martinson wasn’t expecting her mother’s reaction. When Jennifer saw her husband’s dead body, and her daughter with a loaded gun in her hand, she charged at Martinson with a kitchen knife.

Martinson soon overpower her mother, stabbing her repeatedly “with considerable force”.

In the hours after the murders, Martinson took a shower to wash her parents’ blood from her hands and body. She made snacks and drinks for her three younger sisters and locked them in their bedroom.

Martinson, now 18, recently accepted a plea deal for a charge of second-degree murder after claiming mental instability. The court has recognised that she was raised in an unsafe and unsuitable environment.

If found guilty, she could be sentenced for up to 120 years in prison.