If su is compiled with PAM support, it is possible for any local user to send SIGKILL to other processes with root privileges. To exploit this, the user must be able to perform su with a successful login. This does NOT have to be the root user, even using su with the same id is enough, e.g. "su $(whoami)". SIGKILL can only be sent to processes which were executed after the su process. It is not possible to send SIGKILL to processes which were already running.