Key features:

• Customizable Messages:Players tweak the "custom message" part in the format "VictimName was (custom message) by KillerName using [item]" and there is a second optional format which is "KillerName (custom message) VictimName using [item]".
  

• Color Customization: Personalize colors for each part of the kill messages in config.

• Permission Restriction: Optionally limit command use to players with "customKillmessage.use" permission.

• Easy Configuration: Effortlessly change settings and colors in "config.yml."

• GUI:
  if you want to limit the kill messages your players can set,you can turn gui on through the config.this makes it so that they can only use a kill message from the options you have added in the gui(this is turned on by default)
  

Info and instructions:

Instructions:

Use "/setkillmessage "
or if the gui is turned on in the config you can use "/setkillmessage" to set your custom message through the gui.
for viewing all the commands and permissions you can use /cm help(added in 2.0 update)

Tested versions:

I have only tested it in 1.20 but it will probably work in older versions too and if there are any issues let me know here
https://discord.com/invite/ExhzK6Js

Tips:
if you wanna support me,here is one way you can (:
https://ko-fi.com/kespyy