The way the OP presented this question is very misleading.
This is what Dr. Ford actually said:
Such clinical language, I would prefer the more narrative voice:
Bernard walked over to Clementine. He took the pistol from her hand. Overcome with grief and remorse he pressed the muzzle to his temple, knowing that as soon as Dr. Ford left the room he would put an end to this nightmare once and for all.
The narrative happens to be in the past tense.
If in the present tense, the last sentence would be:
Overcome with grief and remorse he presses the muzzle to his temple, knowing that as soon as Dr. Ford leaves the room he will put an end to this nightmare once and for all.
So the reason for having the past tense left and would has nothing to do with "using the past tense for a future time", but has everything to do with the fact that the narrative itself is in the past tense.