Killing processes in Raspbian

The name of kill command family is misleading - they don't really kill anything, just send signals to processes. By default SIGTERM signal is sent, which only gently asks process to quit, but process can choose to ignore it or handle it differently. (see man page for kill).

To force-quit a process, you have to send it a SIGKILL signal. SIGKILL cannot be ignored by the process and in most cases results in its instant termination, without finishing tasks in progress etc.

There are few possible syntaxes to send SIGKILL, all of these are completely equivalent:

kill -9 <pid>
kill -kill <pid>
kill -s SIGKILL <pid>

It's possible that process will be unkillable even with SIGKILL and reboot will be required.