Originally, the kill command/system call just killed a process. This was done by the kernel and the process just disappeared, never being notified about it. That stopped around Third Edition, I think. kill -9 says to send signal number 9 to a process. Unlike most (all? it depends) other signals, it can't be 'caught' by a process and handled in any way. A kinder way to stop a process is kill -15 (or kill -TERM) which tells the process it is being terminated, but gives it a chance to perform cleanup.
Use of kill -9 is a 'guaranteed' way to kill a process; if it's stuck, kill -15 might not always work. Hence, many people still use kill -9 as a 'first resort'.
The reason the 'ultimate' kill signal is number 9 is just the way they did it. There were at least another eight different signals at that time, and I guess the numbers were assigned by the person who programmed that part of the kernel (probably Ken Thompson). Some of the lower numbers are now largely historical, as they map onto hardware instructions and/or events in the PDP-11 hardware. And there are also many others above 9.
Note that the actual numbers have no levels or hierarchy in them; in no sense is signal 8 'less' than signal 9 or 'greater' than signal 7.