Early in the Holocaust, the Germans just had their troops carry out massacres through shooting, stabbing, burning, etc. This caused massive numbers of psychological problems among German troops. Although they believed what they were doing was a necessary evil, they still struggled to cope with that evil. A lot of them fell into substance abuse or ended their own lives.

Enter the death camp. In these, they could have only the most enthusiastic members of the party guard undesirables. You had to be a member of the SS in order to volunteer for guarding a camp. That meant the guards wanted to be there and took pleasure in killing Jews, Roma, communists, homosexuals, and so on. But they still found another way to absolve themselves of guilt.

Inmates were told to take a ticket and remember their number so they could get their stuff back after taking a shower. People voluntarily walked into the gas chambers because it was the same system used for everyday hygiene. Then the SS had other inmates move the bodies to furnaces, with a different set of prisoners loading and unloading. Even when deploying the gas used to kill people, they'd have prisoners drop the capsules into chambers by forcing them at gunpoint. When not gassing people, kapos were inmates used to enforce the rules during downtime. They'd get special privileges like better meals or access to recreational facilities in exchange for snitching and occasionally beating other inmates.

Every step in the industrialized murder machine allowed for nazis to tell themselves they weren't the ones actually killing people. The inmates went into the chamber by their own free will. Other inmates disposed of the bodies. Guards weren't the ones using the gas, they just forced someone else to do it, so that person was responsible. What happened at night in the bunks was the responsibility of the kapos. This is why after the war, they tried to defend their actions with "I was just following orders." They got orders to make prisoners do things, then passed those orders down.

Enforcing and imposing these jobs onto prisoners was done at the hands of only the most bloodthirsty of the SS. They would gleefully torture and murder prisoners on a whim, which made sure the prisoners did what they were told (lest the guard get pissed and decide to torture you out where everyone could see). Other guards only had to watch, telling themselves it was okay because they weren't the ones killing children.

This is how the German people rationalized their participation. Germans outside of the camps only had to report to police or local garrisons when they found "undesirables." Those police and troops only had to load those undesirables onto trains, who took them out of sight and out of mind. Guards then got those people to get off the train and walk right into gas chambers willingly. And this is how nazi high command implicated the entire country in genocide.