Can't guarantee this as an exact science as I suspect there's hidden criteria that can cause on ending to supercede another based on conflicting information I've found all over the place, but I really struggled finding solid information on this online. It seems a lot of people like to refer to the endings as ending 1-4 without specifying what actually happens in that ending to help you track it! I'm hoping my experience can help others complete this game.
Standard ending
Beat game with 3 credits or more. You will likely get this ending without even trying as you play the game. If not, set your credits to 9 and make sure you die a few times (can play on very easy).
Daniel Curien turns back to building and talks about returning to the building if humans go down the wrong path again.
Mystery man ending
Beat game with < 3 credits and an average rank of A. Set your lives to 9 and credits to 2 (can play on very easy).
Select "entrance" (left) option in chapter 1. Then always select left option when given a choice. This route will lead you to The Fool (sloth), Sun (plant), Death (security guard)
I personally have received this ending when taking other routes, e.g. facing The fool (sloth) last. My guess (and this really is a guess) is that higher ranks (lots of S ranks) and successfully saving your partner every time can cause this ending to supercede the one you would otherwise get for the route you took.
Mystery business man picks up a vial and says: "It appears that he didn't understand its true purpose"
Zombie drives car away ending
Beat game with < 3 credits and an average rank of A. Set your lives to 9 and credits to 2 (can play on very easy).
Select "entrance" (left) option in chapter 1. Then select your options so that BIO LAB is last and you fight Sun (plant) boss as your final selectable boss.
Lisa thinks her dad is driving away in the car but her dad calls her from behind and camera pans to a zombie driving away their car
Daniel turns in to zombie ending
Beat game with < 3 credits and an average rank of A. Set your lives to 9 and credits to 2 (can play on very easy).
Select "parking" (right) option in chapter 1 (this was the critical thing that appeared to help me finally get this ending). Then select options so that Death (security guard) is the last selectable boss you fight. My route was The Fool (sloth), Sun (plant), Death (security guard).
Daniel goes crazy talking about not having any purpose in his life now. He becomes a zombie and Lisa is heard screaming
This is the ending I really struggled to get and didn't find any information matching what I did to get this ending.
Additional info about my run that may help you:
- I failed 1 time to rescue my partner
- I Ended the game with "EXPERT AGENT" ranking with ranks: A B A S A A