For each of the key Normandy upgrades you haven't completed, someone will die on the way to Collector Base.
If you choose someone other than Legion/Tali/Kasumi to open the doors into the station, if you choose someone other than Garrus/Miranda/Jacob to lead the diversion, or if you didn't the loyalty mission of either; whoever opens the doors dies.
If you choose someone other than Morinth/Samara/Jack to project the biotic field, one of your squadmates will die. (There's an ordered list; according to the research I just did, getting Zaeed killed here requires your squad to be Zaeed and Morinth).
If you send someone who's not loyal to escort the crew back to the Normandy, the crew will live but the escort will die. (If you don't send anyone, the crew will die.)
If you choose someone other than Miranda/Jacob/Garrus to lead the diversion at this point, or if you choose Jacob/Garrus without completing their loyalty mission, the diversion leader will die. (apparently there's also a special case where they'll survive if you only have four squadmates alive; I believe this and Miranda's loyalty exemption are there to preserve mission/dialog flow)
If you choose any squadmates for fighting the final Reaper that you haven't completed the loyalty mission for, they'll die at the end of the fight. (This is the most reliable option for Zaeed's death, though of course requires you not to have done Zaeed's loyalty mission....Actually I think you can complete the mission
without gaining his loyalty? That might work...but again requires setup on your part)
And then the complicated part....The unlisted "combat rating" of all the characters
not in your squad for the final battle is used to determine whether any of them die. Basically, assuming I remember the numbers right:
- Start with 0.
- For each of Garrus/Grunt/Zaeed, add 2.
- For each of Mordin/Tali/Jack/Kasumi, subtract 1.
- For each person who hasn't had their loyalty mission completed, subtract 1.
If this is less than zero, one or two people die (again, there's an ordered list; and as Zaeed is second-to-last of it you'll almost certainly never engineer his death here).
For this reason, it's strongly suggested that Mordin/Tali/Jack/Kasumi (if loyal) are the one sent to escort the Normandy crew back, and not to take two of Garrus/Grunt/Zaeed with you into the final battle.