The Mastermind skill tree is for players who want to have more crowd control. Skills here are tied to keeping the civilians under control and generally keeping a handle on the whole heist. Good skills to unlocked here include...
...Cable Guy. This allows you to zip-tie civilians quicker. The faster you move, the quicker you'll have hostages.
...Pistol Messiah. Unlock this, and killing an enemy when downed will revive you.
...Combat Doctor. This gives you a second Doctor Bag, essential for keeping your team alive.
...Inspire. This speeds up reviving team members, also important for keeping everyone alive.