Except my first play through where I wanted the achievement, I kill off Jacob every time. Others are random depending on Shep's mood. Jacob is just not going to make it to ME3 in my games, I'm not even going to use my everyone lives ME2 ending as an import. Here's why I kill him, he reminds me of Kaidan. After Horizon, I really don't want Kaidan anywhere near me but it seems like I will have no choice because like Jacob, I always killed Ashley in play throughs. Still wishing I could have offed both Kaidan and Ashely and got to keep Kirrahe. Jacob always falls during his own loyalty mission. Really? You dragged me here and you can't keep from falling. I also believe in not using medi-gel, too bad for you if you fall. Then he volunteers to go through the vents during the SM, well everyone in that room knew it was not a good idea. I still wish Miranda didn't interject and it was Legion instead who would say, "Odds of survival 0%." So, he goes through the vents every time.
I've killed almost every character in the dozens of endings I have, the only one that has survived every time is Thane. Yep, never killed him. The irony of the dying one always getting to live. And well, he's an assassin. I love assassins.