Those people are lunatics.
I'd rather not play a lunatic. I know what I'm going to do moment-to-moment. If you don't, I'm legitimately afraid of you.
(let's be honest - in real life I'm afraid of everyone)
but it's most people that are like that at some point in their life. Or sometimes we do something we knew we were going to do, but didn't think it was something we would be able to go through with.
We've already been down this discussion before and you've admitted you don't understand and don't care to understand Human psychology. I'm not sure why after that I should believe your word on who is or isn't a lunatic, because you clearly don't know what your talking about by your own admission.
What about when they're not desperate?
And shouldn't the player be the one to decide when the character is desperate? Even if the character were to act in a way that he couldn't have predicted in advance, the player should still know what's going to happen because the player is the one pressing the buttons. The player should have better knowledge about the character than the character has about himself.
It's up to you to come up with the reason as to why. I've already noted there is enough ambiguity for you to work with here.
Desperation was just one possible answer. It's not my job to list all of them for you.
You already have to headcanon 100% of this scenario in KotoR, so doing the same in Mass Effect shouldn't be a stretch if KotoR isn't.
No, it's not particularly helpful when either side does it =P