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 -xf 'valgrind --tool=lackey ./testcases/kernel/syscalls/waitpid/waitpid03'
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 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).