I don't feel like this is worth opening a separate issue, I noticed that putting kill as the last thing in the file (so [ k ][ i ][ l ][ l ][EOF] - the problem doesn't happen when there's a space or a line break after it) highlights it as a syntax error:
From https://github.com/Arcensoth/language-mcfunction/issues/52#issuecomment-890549834 :