Were there any devout Christians among the Nazis who participated in the Holocaust? How did they come to terms with the things they had done? How did they justify their actions in front of the altar of God?

Not sure if this would be considered devout. Josef Tizo was a Catholic priest and also one of the leaders of the Slovaks Peoples Party a Clerical-Fascist party that was installed in power during WW2. It was also founded by another Catholic priest Andrej Hlinka, for which their paramilitary, the Hlinka Guard was named.

The party ruled from 1939-1945, pretty much the duration of the war. And its record is not pretty. The party not only acquiesced to every single Nazi request, but participated enthusiastically. This included the "Final Solution", the party actively used the state to find and deport Slovakia's Jews to the Polish death and concentration camps in 1942. This was on top of variations of the Nuremburg Laws (The Jewish Code as known locally) that had been enacted in 1941.

The leadership of the Catholic church was supportive initially, an apostolic blessing from Pope Pius XII was given to President Tiso after his ascension. They were not so pleased as the antisemitic nature of the clerical-fascist regime took hold. In general, Tiso was supported by large numbers of ordinary Catholic priests and deplored by the bishops and the Vatican itself. Though there are lots of exceptions, such as Bishop Jan Vojtaššák, who thoroughly supported Tiso and the persecution of the Jews.

While opposed by the Church, the Vatican made no effort to excommunicate or restrict Tiso (or his prime minister, also a Catholic priest).

After the war, Tiso would be tracked down and arrested by the American occupying forces and handed over to a judicial tribunal in Czechoslovakia the Americans had organized. This was prior to the takeover by the Communists. Tiso was condemned to death and executed in 1947 for his crimes against Slovakia's citizens, including Jews.

In 1995, the Catholic Church did a good job of offending Jews when Pope John Paul II proposed beautifying previously mentioned Tiso supporter Bishop Jan Vojtaššák for his opposition to the Communist state. Despite that Jan Vojtaššák had not found a persecution of Jews he could not support.

Some Sources: Dr. Pavol Mest’an, Slovak historian, director of Museum of Jewish Culture of Bratislava

The Vatican, the Catholic Church, the Catholics and the Persecution of the Jews during World War II: The Case of Slovakia in Jews and non-Jews in Eastern Europe by Yeshajahu Jelinek, University of Minnesota, 1974 (at the time, he jumped back to Israel's Hebrew University after his stint in Minnesota)

There is another book specifically on Slovakian Jews was contributed to by Yeshajahu Jelinek, The Tragedy of the Jews of Slovakia: 1938-1945 Slovakia and the Final Solution of the Jewish Question, published in 2002 I have yet to read.