It’s fair to say that the ending of Mass Effect 3, and so BioWare’s entire Mass Effect trilogy, remains divisive to this day – and that’s in large part due to the child-like Catalyst. The Mass Effect 3 original ending, however, featured a god-like Reaper Queen instead – and a possibly straight Paragon or Renegade choice.
Mass Effect 3’s current ending features players having to make a choice between three possibilities – Destroy, Synthesis, and Control – as orchestrated with the consciousness of the Reaper architect, the Catalyst. Destroy wipes out the Reapers, all synthetic life, and possibly Earth. Control sees Shepard merges and controlling the Reapers. Synthesis fuses organic and synthetic life galaxy-wide.
According to former BioWare animation director Dave Wilkinson in an interview with YouTube channel People Make Games (via GamingBible), it wasn’t always like this. Originally, Shepard – who was supposed to become a Deus Ex-like augment – plugged himself into the Citadel and had a conversation in virtual space with “the god of the Reapers” or a Reaper Queen.