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It was only with version 7 of Unix that signal 15 was introduced (see see the signal(2) man page for v7) and kill switched to that rather than using signal 9.

It was only with version 7 of Unix that signal 15 was introduced (see see the signal(2) and kill(1) man pages for v7) and kill switched to that rather than using signal 9.

By the time sixth edition had come around, the kill command had already been overloaded to allow any signal to be sent.

From the manual page from version 6 of Unix:

If a signal number preceded by - is given
as first argument, that signal is sent instead of
kill (see signal (II)).

The default though was to send a signal 9, as signal 15 did not yet exist (see the signal(2) man page for v6).

By the time fifth edition had come around, the kill command had already been overloaded to allow any signal to be sent.

From the Unix Programmers Manual, Fifth Edition (p70):

If a signal number preceded by "-" is given
as an argument, that signal is sent instead of
kill (see signal (II)).

The default though was to send a signal 9, as signal 15 did not yet exist (see p150).

With version 6 the kill man page no longer mentioned the same typewriter bug.

Originally, the kill command could only send a signal 15 (see UNIX PROGRAMMERS MANUAL Seventh Edition Volume 2A, January 1979 pages 48 & 100), which does attempt to kill the process (but can be ignored by the process), but these days kill can be used to send any signal.

The default is still to signal the process in a way which can caught and handled gracefully or ignored (by sending a SIGTERM signal), but it can also be used to pull the rug out from under a process (a kill -9 sends a SIGKILL signal which cannot be caught and thus cannot be ignored).

The main metaphor for processes is that of a living being which is born, lives and dies.

Thus, the kill command fits in with this metaphor.

Originally, the kill command could only kill a process, only later was kill enhanced to allow you to send any signal.

Since version 7 of Unix (1979) the default has been to signal the process in a way which can be caught and either handled gracefully or ignored (by sending a SIGTERM signal), but it can also be used to pull the rug out from under a process (a kill -9 sends a SIGKILL signal which cannot be caught and thus cannot be ignored).

The main metaphor for processes is that of a living thing which is born, lives and dies.

Thus, the kill command fits in with this metaphor.

Unix Archaeology

From the manual page from version 4 of Unix (the version where kill was introduced, along with ps) we find:

NAME
        kill - do in an unwanted process
SYNOPSIS
        kill processid ...
DESCRIPTION
        Kills the specified processes.
        The processid of each asynchronous process
        started with `&' is reported by the shell.
        Processid's can also be found by using ps (I).

        The killed process must have
        been started from the same typewriter
        as the current user, unless
        he is the superuser.
SEE ALSO
        ps(I), sh(I)

I particularly like the final section of this man page:

BUGS
        Clearly people should only be allowed to kill
        processes owned by them, and having the same typewriter
        is neither necessary nor sufficient.

By the time sixth edition had come around, the kill command had already been overloaded to allow any signal to be sent.

From the manual page from version 6 of Unix:

If a signal number preceded by - is given
as first argument, that signal is sent instead of
kill (see signal (II)).

The default though was to send a signal 9, as signal 15 did not yet exist (see the signal(2) man page for v6).

It was only with version 7 of Unix that signal 15 was introduced (see see the signal(2) man page for v7) and kill switched to that rather than using signal 9.