SIGKILL (kill -9)

Signal 9, the one signal that is not a message: it is never delivered to the program at all. signal(7) states that SIGKILL cannot be caught, blocked or ignored. The kernel takes the process off the run queue and reclaims its memory, and the last instruction it ran is whichever one it happened to be on. So -9 is not "kill harder", it is giving up on the clean stop, and you pay for that: bytes still sitting in a buffer never reach the disk, and the lock file or PID file the program would have removed stays behind, so the next start fails with "already running". It is not all-powerful either. sudo kill -9 1 does exactly nothing, a zombie has already exited, and a process in D state — uninterruptible sleep — will not move until its storage answers.