[spoiler] um...Could we not just have come back for him?

[Drzappleswag]

Simon 3 doesn't have the capacity to rescue Simon 2. He's in a Pilot Seat at the bottom of the Abyss.

He can't backtrack because half way between Tau to Omicron has no lights.

Furthermore they certainly can't just climb up to the surface. They're robots. They can't swim. They are stuck at the bottom of the Sea.

I guess theoretically Simon 3 could risk his life and potentially rescue Simon 2 but that isn't a plot hole. You were given the choice to kill him. If you did that isn't the developers fault.

You made the rash decision based on false information to kill Simon 2. If you feel like you screwed up because you didn't know better at the time, it's no one's fault but your own.