My STRONG advice:

Get an electric saw.

They're light (mine can't be more than 9 lbs), there's zero burn risk, and aside from the cord there's really no downside.
You never have to worry about starting it, it's just safety-trigger-VRNNNNNN! All night!

Plus, because there's no real risk of burning anybody. When a group of screamers comes through, I'll just walk around the corner with the saw out (not even on!) and slam it on the wall next to them or even above their heads (they drop to the floor EVERY time). I get the saw RIGHT IN THEIR FACE (again no heat so no burn, and it's not on so no moving parts), I give them not one inch of personal space. I've had people in the corner of my room screaming for up to two minutes without even touching the safety!
Then when they finally get free, I start the saw and the chase begins!

The most important things to remember about acting with a chainsaw are that people are more scared of the anticipation and the build up then they are of the actual saw, and you can't let the saw do the work for you.
Sure, you could just run out of the dark with the saw full on. And yeah, that'll get maybe 40 or 50% of groups a good scare, but why do that when you can make them scared of YOU, not just the saw!?