This sounds like the start of some "moral lesson" cartoon on PBS but instead, an owner's wandering cat has received a death-threat. From the BBC:
Smudge the cat disappeared from his home in Huntingdon, Cambs, two weeks ago. He was found five days later with a note placed under his collar.The letter, addressed "To whom it may concern" said: "Keep your cat indoors... If I catch it again it will be for the last time."
A Cambs Police spokeswoman said they were looking into the incident.
[...]
The family said eight-year-old Smudge was found with the note on a piece of paper torn out of a notebook tucked under his collar.It read: "Your cat 'Smudge' keeps coming into my home, eating my cats' food and fighting with my cats.
"I'm sick of it bullying my cats and causing fights at stupid hours of the late evening/early morning disturbing my sleep.
"I'd advise you strongly to keep your cat indoors in the evening. If I catch it again it will be for the last time and you will not see Smudge again."