There are two different endings in game and both need to be unlocked for the 'End of Night' and 'Just A Memory Now' trophies. When you have defeated the huge mutant boss you will need to make a choice on whether to cure Mia or Zoe as there is only enough serum to cure one of them. Curing Mia gives you Ending #1 (Good) and curing Zoe gives Ending #2 (Bad).