I do not believe when you say you didn't see him, as you were looking at him. And you can easily check if he was an alpha zombie as there were only like 15-16 players at the time. It is easy to see if he was starting zombie or not.
As for the slaying yourself, you clearly died on your own to avoid a zombie tag. You were shooting and moving and died all of a sudden? do you have sm_slay @me binded or something? Cause I don't see anything on console or typed..

Even though you are an admin and know the rules better, you do not have any previous bans on record, and protocol must follow.
I'm voting warn, and a final warning if others agree with me.