What if I just use userland kill(1) utility? Is it possible to kill all processes under a PGID using kill(1)?

Say the PGID I get for sh is 321. If I do

    kill [signal] 321

that will not kill all the processes having PGID 321?

If it would not kill them, then couldn't we modify kill(1) to be able to call kill() with a negative integer as you describe?

Sorry for the noob questions. I am still learning and making mistakes.