I'm sorry this happened to you, but it highlights another very important factor. Don't keep all keys to the kingdom on one person. Always divide and conquer. Keep power distributed between multiple people. I worked at a company of 500+ people, and I'm sure the CEO didn't have access to all the IT people's stuff. They only cared that everything works and meet their quarterly goals. Shall the IT person feel like sabotaging stuff, there are distributed backups and mainly the fine print in the work contract preventing that.

I know this doesn't necessarily apply to smaller companies and startups, but have lawyers write you strong contracts that aren't one-sided, but are full of protections for both sides, if they aren't sabotaging stuff.