A Wisconsin woman accused of abusing and killing her boyfriend's dog allegedly kept a diary in which she detailed the "pleasure" she gets "from watching her whimper in pain and cry out for help."

Sean D. Janas, 20, is accused of mistreating, poisoning and killing a four-year-old shepherd-Labrador mix belonging to her live-in boyfriend, Steven J. Kuick.

The investigation began when the downstairs neighbour of the Marathon County, WI, couple reported to police he'd heard Janas yelling at the crying dog on June 4, then found a puddle of blood in the backyard later that day.

Police say another neighbour told them he'd seen Janas strike the dog with a leash repeatedly, then stab it with what looked like a knife. Soon after, he'd heard from the boyfriend the dog was dead.

Police say when they investigated the suspect's apartment, they found blood on the steps and garage, blood-soaked blankets and what appeared to be a dog sweater full of blood and holes.

When questioned by police, Kuick said he'd come home to find the bloodied animal dead on the step. Janas said she put the dog outdoors. Later, she said she heard him yelping and found him dead.

A vet confirmed the dog had lacerations all over its body from a knife, and appeared to have been poisoned, police said.

A criminal complaint obtained by ABC's Wisconsin affiliate quotes from a diary Kuick's father says he found while cleaning out the couple's home.

Janas allegedly wrote: "I need to find a way to kill her without it looking like I killed her. I've done lots of things already. I've given her drano bleach & a lot of pain pills lol one night she got all tweaked out because of it, it was so funny. I beat the living s--- out of her a few days ago & shes got a huge like sack on her head and she won't stop puking on everything. So imma give her more pills & hopefully that dumb bitch dies. Because Steve thinks she ate something outside & that's why shes sick.

I just wanna beat her till she f------ dies. Which I mean I have beat her I mean. I tell you I don't care how long it takes I'm just gonna keep feeding her pills & s--- till she croaks because shes already sick & we don't have any money to take her to the vet so hopefully she just dies so I don't have to ever see her dumb ass again. I have never hated an animal so much in my life, much less hate one or beat one. But the pleasure I get from watching her whimper in pain & cry out for help as I shove drano & bleach down her throat is like no other."

If convicted, Janas faces more than five years behind bars and $30,000 in fines.