killall valgrind will kill all valgrind processes regardless of arguments. If you want to kill only processes whose command line is exactly valgrind --tool=lackey ./testcases/kernel/syscalls/waitpid/waitpid03, you can use pkill:
pkill valgrind --tool=lackey ./testcases/kernel/syscalls/waitpid/waitpid03
Like killall, pkill is on every non-embedded (and some embedded) Linux installations, and it's more powerful and often more reliable (but for some reason less well-known). The companion utility pgrep is identical except that it lists the PIDs instead of killing.
Another utility you may be interested in is fuser: fuser testcases/kernel/syscalls/waitpid/waitpid03 lists the processes that have the specified file open, and fuser -k would send a signal to these processes. When you're not trying to send a signal, lsof is a more powerful alternative to fuser (shows more stuff, has more filters).
-9is a last resort. You should use-SIGTERM,-SIGINTor-SIGQUITfirst to give the application a chance to do cleanup and exit gracefully. – Dennis Williamson Dec 10 '10 at 16:51