Also doesn't Orlock say something along the lines of "I don't understand why Visari trusted you"?

He kills the Helghast outside of the building to cover up that Visari had a double there, that way only he would know and it would give him a chance to take over while Visari is away.

I am guessing the others would be clueless about the double? Why else would you kill that soldier after Visari enters the building?

Radec would have obviously been in on it, but on side with Stahl, so I am wondering why he stayed behind to get killed. The obvious intent there would be to have the double get killed and/or captured and the real Visari in hiding.

I really want it to be Visari, but Radec is the part that is confusing me.

maybe that wasn't really radec they fought at the end? Hey what about the traitor from killzone Liberation?

I doubt it, but if Radec is alive too I will jizz.

I also never played Liberation. I haven't even played the original Killzone.

neither have I,  looked it up on wikipedia. When I think about all of your points it really makes sense, I was wondering why Radec shot that soldier but hell Darth Vader treats his troops better than the Helghast commanders lol

I could mybe see Visari being alive but Radec shot himself in the head. He isn't alive. Lol.

But what if that guy wasn't radec? What if it was his double, don't understand why Visari would leave without some protection.

Your thinking too much- Visari wanted to crush the ISA, and he wasn't about to let something like the possibility of death get in the way. 

The possibility of a double is making me curious. The reaction from his daughter bugged me.

There are some people saying that it can't be Visari because when the mysterious man stepped out of the vault they addressed him as "Sir."

Well at the end of the opening cinematic they addressed Visari as "Sir" when they told him the ISA was there.