Why is there a /etc/init.d/mysql file on this Slackware machine? How could it have gotten there? [closed]

Solution 1:

Your question is impossible to answer. Perhaps the previous admin installed the MySQL with some other way than the official one? Perhaps he just googled for the init script and manually copied it to /etc/init.d?

There's no way to know what happened, unless you have some kind of audit logs.