How did the Jews at concentration camps view their existence when death was a probability?
I am not sure anyone can answer this with great certainty unless they were there to experience such a reality. The other way this question can be answered is through survivor testimony or reading the thoughts who were there. This would be where sites like YouTube are so important in that we can seek out survivor testimony and learn from it. We, as modern individuals, can learn how consciousness functions under the looming shadow of death at any time. How does random selection and a sick sense of probability figure into living one's life? How does one love, enjoy life, or see "another butterfly?" The other option would be to read works from those who endured the experience. Of course, one cannot go far here without reading Elie Wiesel's narrative, "Night," which talks about how a young boy, Eliezer, endures life at the camps with death as something that is real. It shows how the stripping away of his humanity was part of this equation, renouncing community, spirituality, family, and hope in order to live. Another step in this process would be to examine writings from and inspired by The Holocaust. There is a great book of poetry entitled (appropriately enough), "Holocaust Poetry" that features many poems which deal with how one lives life in the face of constant death. It is a wonderful read that brings out the pain and sorrow, the agony as well as the hope of redemption that are all intrinsic to any worthwhile study of the Holocaust.
As the previous poster said, it can be pretty tough to answer this question. A great place to look for a personal discussion of those feelings would be in Gerda Weissman-Klein's books or documentaries. She was certainly surrounded by the death of her whole family and had to know that her own death was likely if not imminent.
She discusses at great length how important very mundane or ordinary things became when faced with such things. She has an incredible ability to tell the story of what was happpening and still finding beauty and hope in the world around her.
But again, instead of trying to answer the question myself, I'd suggest you try to find the answer in those places or in other personal testimonies, etc.