HENRYETTA, Oklahoma - Police say a Henryetta man was killed by his off-and-on again girlfriend; one investigator said it's the most brutal crime he's worked.

The victim's family said they knew the relationship was toxic, but never saw it ending in murder.

Police said the victim's girlfriend, Samantha Stansifer, told them she killed her boyfriend, Anthony Hall.

Now Hall's sister, Amanda Feldpausch, is now on a hunt for justice, which began Thursday as Stansifer made her first appearance in court.

Feldpausch said she wonders how her brother's 3-year relationship with Stansifer went so wrong.

"Worst thing ever imaginable," she said.

An arrest affidavit says the couple was at a party when Stansifer noticed text messages from another girl on Hall's phone.

The two went back to Hall's house and were falling asleep, when Stansifer noticed more texts from the same girl.

The affidavit says Stansifer went to get to the kitchen to get a drink, noticed two knives and came back stabbing and mutilating his body.

Henryetta Police say Stansifer called 911, and when officers found her she was drenched in blood, telling them she killed Hall.

"As his sister, I feel like, at this point, he's not here. The only thing I can do for him is come here and make sure she gets what she deserves," Feldpausch said.

Donna Mathews, with Domestic Violence Intervention Services, said she's seeing a new domestic violence trend – fewer women are being killed by men, but the number of women killing men isn't decreasing.

"What can we do to screen for a woman who's likely to do that, just like we might screen for a man who's likely to do that," Mathews asked.

Feldpausch said her brother told her he tried to break it off.

"I said, 'Good, I think it was a toxic relationship.' And he said, 'Yes, I know sis, you're right. I should've done it a long time ago.' So he knew she wasn't good for him, but she was his first love. I think that was his first ever real serious girlfriend," she said.

Now her brother, who she describes as a good kid, won't have a chance at another relationship.

"I'm just heartbroken and wish this wasn't true," said Feldpausch.

Stansifer's attorney declined comment Thursday in court.

Stansifer was also arrested for domestic abuse assault and battery in 2013.