The SS did not send prisoners on death marches to murder them on the road. It had other motives for the evacuations in spring 1945, all of them utterly deluded. SS leader Heinrich Himmler wanted to hold onto prisoners, especially Jews, as bargaining chips for peace negotiations with the Allies (in reality, the Allies refused to negotiate with Himmler). The SS also claimed that leaving prisoners behind posed a terrible threat to local civilians (in reality, there was no such threat). Finally, the SS planned to redeploy evacuated prisoners as slave labourers for the war effort (in reality, war production had already largely collapsed). The SS delusions are summed up by the former Auschwitz commandant Rudolf Höss, writing in Polish captivity after the war.
I was having serious doubts about whether we could win the war! I had seen and heard about too many things that were going wrong. If this went on we wouldn’t be able to win. But I couldn’t allow myself any doubts about our final victory; I had to believe in it. Even if my common sense was telling me clearly and unequivocally that we were bound to lose. I was devoted to the Führer, to our ideology, they couldn’t be allowed to fail. […]
We had to win
Our world couldn’t be permitted to collapse. We had to win. Every one of us worked fanatically as if victory depended on our work. Indeed, when the Oder front collapsed in April  we took great pains to keep the remaining armaments plants producing at full capacity by using prisoners. No effort was to be spared. In fact we even considered whether we could organize emergency armaments production in the temporary camps, which were beyond primitive. Anybody in our organization who did not bother to do something, saying that there was no point any more, was severely reprimanded.
Source: M. Broszat (ed.), Kommandant in Auschwitz (Munich, 1994), pp. 217–18 (emphasis in the original)
Translation: Lesley Sharpe and Jeremy Noakes