See what happens when you kill PID 1 on a system using sysvinit.
The only signals that can be sent to process ID 1, the init process, are those for which init has explicitly installed signal handlers. This is done to assure the system is not brought down accidentally. (From kill(2))
In practice, you will find that:
kill(1, SIGKILL) = 0
someapp &
[job 1 backgrounded]
kill %1
(job 1 is killed - yep, you can use kill for jobs)
Probably because you know the features of your shell and enjoy productivity.
zx2c4@thinkpad ~ $ sudo strace -p 1
Process 1 attached
select(11, [10], NULL, NULL, {1, 122329}) = 0 (Timeout)
stat("/dev/initctl", {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
fstat(10, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
stat("/dev/initctl", {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
select(11, [10], NULL, NULL, {5, 0}) = 0 (Timeout)
stat("/dev/initctl", {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
fstat(10, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
stat("/dev/initctl", {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
select(11, [10], NULL, NULL, {5, 0}^CProcess 1 detached
<detached ...>