I beilive this is a feature request, because KILL works as documented.

With KILL <thread_id> one can pass an expression:

MariaDB [(none)]> SET @x = 100;

Query OK, 0 rows affected (0.00 sec)

MariaDB [(none)]> KILL @x + 1;

ERROR 1094 (HY000): Unknown thread id: 101

But with KILL USER you can't. I'm writing a Stored Proc where I may want to execute KILL; Prepared Statements are a good workaround, but passing a variable could be much less verbose.