I've changed the config, even removed the kill notification, and when I reload the plugin, it resets the config to default.

If it resets the config to default, then what you put in isn't the right format, so the plugin rewrites it back to default settings.

See change notes for version 6.4.9 for additional details, but to make a long story short, don't delete the default death messages. Just disable them by setting Enabled to false if you do not want them to activate. You may also add new death messages if your syntax is correct. I suspect you're deleting them and they're getting regenerated when the plugin starts up.