Process with Command 'Killed' and State 'Executing'. Not getting killed

Last day, when I ran a count(*) query on a table. It didn't provide any results. It ran forever even. So I tried killing those queries (I have tried it multiple times). As I'm using AWS RDS, I killed the session using > CALL mysql.rds_kill(pid); When I checked the processlist, it was showing **Killed** in the Command column and **Executing** in the State column. Why is it now killing it? The main issue is, the CPU is hitting 100% from the time when I ran this query. Even after I ran this kill command, it stays at 100%. As I'm a newbie in DB, I would like to know why it was not getting killed fully. I have read about the time taking for rollback even after we kill a process. But here, I just tried a count(*) query. Where can I find details regarding this? Right now, I have rebooted to bring down the CPU. But if this happens in production, things will get messed up, right? Any help for getting details regarding this issue would be really helpful. Thanks.