You don't need to kill them.
To build up some basic understanding:
Every process is the child of a parent process (except for the init process, but we don't need to worry about that).
When a child process finishes executing, it might still contain data that the parent process wants to access, usually an exit code which describes the result of the child process (Did it succeed? Did it error? What was the error code?).
A finished child process hangs around in the process table as a zombie process until the parent process "reaps" or waits on the child process, which is simply the parent process telling the OS that it doesn't need that child process anymore; It has read any return values it wants, and is done with it. At this point, the system will clean up the entry in the process table.