You ever read the expanded universe series of Star Wars books?
You had a kid that became literally one with the light side of the force (a living conduit) then he becomes a sith lord soon after.
Exar Kun made more come backs than Ric Flair.
Chewie got a freakin MOON dropped on him
and Leia whom had been inflicted with a Poison that nobody else had survived and there was no antidote for, purged it from her system because Force User.
KOTOR II isn't all that unbelievable compared, at all.
The EU is generally crap.
Every single one of the 3 main bad guys is completely unbelieveable. Let me name the ways:
1) Being able to survive being decapitated and cut into tiny pieces. That's impossible. You'd be killed and turned into a force ghost according to Kotor 1. While the Force was able to slowly restore inanimate objects in Kotor 1, that process took thousands of years just to restore a star map or human sized droid. And even then, it was a combination of the Force and nanobots. That wasn't even really a big deal since normal droids could also repair themselves even faster without using the Force (provided they hadn't been destroyed).
2) Being able to use the force to destroy an entire planet of force users. That's impossible. The force has never been portrayed as being able to perform feats like that, in fact it has been portrayed in quite the opposite fashion in Kotor 1.
3) Being able to use the Force to destroy the Force. That's impossible. That also implies that the Force is something other than the Force. In the Star Wars episodes 4-6, the Force is portrayed more along the lines of being or existence itself, the thing which composes the universe. Kotor 1 doesn't really dispute that concept. Even episodes 1-3 tend to hold a similar view, but living beings are only able to interact with the higher plane of existence in so far as they have undefinable midichlorians in their bodies. In order for the Force to be destroyable, it would have to be a discrete existence, which doesn't really mesh with how it is portrayed.