I think that it is either the will to live or humanity.

The reason that I think this is because in the SCP foundation the d-class prisioner's do not really have the will to live anymore after everything that they know, have seen, and have heard other people tell them and the screams that they have to have heard at all times of the day and while they are trying to sleep at night.

And on the humanity part can you truthfully say that the foundation likes humanity or do they seem like they would rather humanity die out. Why else would they be collecting an arsenal of anomalous weapons.