Forensic Files: "Dirty Little Secret"

A couple that goes on vacation does not come back.
They met another couple while on vacation, who were killers.
The couple was shot, and stabbed. The female killer got a tattoo on her abdomen where she stabbed her female victim. They killed the couple just for the thrill of killing. They were hunting for thrills. They asked the victims to come to their hotel where they toyed with the victims, until they killed them ultimately.

Forensics found
1. A palm print in the bathroom window.
2. Blood on the bathroom floor, even though the killers cleaned up
3. Flesh and Blood in the drains