Police are investigating after one cat was found with its ears and tail cut off and another was stabbed in the back.
Felix, who was found with a slice down his spine in Snodland, Kent, South East England, was later put down when taken to a vet.
Storm, who had been missing for 17 days, was found dead in the same woodland with missing body parts.
Tessa Gould, 49, said her son Michael, who owns Storm, is heartbroken.
Tessa said: "She has had her ears and tail cut off. Someone out there knows the monster that did this so please get in contact with the police.
"My son is heartbroken and I'm ready to inflict a lot of pain on whoever did this."
Tessa has called for people in Snodland to be aware as she "wouldn't want this happening to anyone else".
She added: "The people who examined her are hopeful she passed away before they mutilated her tail or ears.
"It's not much, but at least it's something. You just have to hope she didn't suffer.
"There is obviously a monster out there doing this. To hear your son crying down the phone is awful.
"He's just heartbroken. He's full of questions, but there's nothing you can do. He just wishes he had never let her out, but cats are cats. They get outside."
Michael, 27, said: "I saw a post online saying a dead cat was found in Sandy Lane which is close to where I live and walked down there hoping it wasn't her.
"I was thinking no way can it be her, she's too quick and too smart to get killed but then I saw her laying there and burst into tears. I haven't slept in days.
"Her ears and tail were cut off to the bone. It was horrible.
"I couldn't believe it had happened. I wouldn't have been as surprised if she was hit by a car as we live on a main road, but never expected this.
"It just shows what a crazy world we live in and how sick some people can be."
Kent Police confirmed they are treating the killings as being intentionally caused by a person.
Sergeant Sarah Alvey said: "We understand that hearing of these incidents will cause concern for cat owners but please be assured we will follow up on any reasonable line of enquiry that is reported to us.
"Our neighbourhood officers are also aware of the incidents and will remain vigilant for offending of this nature while they complete their routine patrols.
"If there is anyone who believes they have information that can assist with enquiries, I would encourage them to contact us at their earliest opportunity."
Following the attacks, members of South London Animal Investigation Network (SLAIN) are patrolling the area.
The group will hand out leaflets on Monday appealing for witnesses and CCTV.
Anyone who finds another mutilated animal in the area can get in touch with SLAIN via its Facebook page.