He suspected that Nazi rule and government would result in crackdowns on Communism within Germany and Martin could only see this as an unqualified good. He was definitely uncomfortable with some of the consequences of Hitler's rise to power but he was apparently willing to put up with the negatives for the chance to punish those he disagreed with.
Then they came for the Jews and Martin didn't speak up because he wasn't a Jew. He was the son of a Lutheran pastor and had served in World War I aboard various U-boats. He was part of crews that flew a false French flag and sank British and American ships in an attempt to shut down naval commerce in the Mediterranean Sea and the Strait of Otranto. After World War I finished, he decided first to become a farmer. He got married and had very little success as a farmer. Eventually, he gave up farming and decided to follow in his father's footsteps by first studying how to be and then becoming a pastor in a Lutheran congregation.
Then they came for the trade unionists and Martin didn't speak up because he wasn't a trade unionist. Instead he was a pastor who hoped to use the Christian faith to apply order to society and restrain cultural evils. He was looking to unite people with the power of religion but it's unclear about the state of his own faith at the time. He professed belief in the Faith of his fathers but it seemed that Christianity was nothing more than a tool to attain some particular interpretation of utopia within society.
Then they came for the Catholics and Martin didn't speak up because he was a Protestant. In fact, he was a leading Protestant who had an opportunity to speak on behalf of other Protestants in an audience with Hitler. Hitler insisted that he wanted the support of the Protestants. Martin would write many years later: "Hitler promised me on his word of honor, to protect the Church, and not to issue any anti-Church laws. He also agreed not to allow pogroms against the Jews, assuring me as follows: 'There will be restrictions against the Jews, but there will be no ghettos, no pogroms, in Germany.'" Still hoping to purchase security and safety by sacrificing others, he was willing to make this deal because he suspected it would be good for the Church--he was so very wrong.
Then they came for Martin and by that time no one was left to speak up. When Hitler began oppressing the parts of the Church that Martin was associated with he was forced to come to a sudden realization--he had been dealing with the devil and selling his soul for a little more security. He became an outspoken opponent of Hitler and the Nazi regime but there was little time left to change the way of things. He was arrested and held in prison for resisting and opposing the Nazis. When he was released, he was picked up by the Gestapo under the direction of Rudolf Hess. He was sent to concentration and works camps--both Sachenhausen and Dachau. As the Allied forces were liberating the camps, he was transferred to Tyrol and eventually set free. He had suffered but most importantly he had changed. He has been converted from the gospel of security through control and to the Gospel of Jesus the Slaughtered. For years he devoted himself to making up for his earlier failures and never once failing to admit his guilt. He became a pacifist and a proponent of nuclear disarmament and led congregations to work together to expand the Kingdom of God and not simply to endorse or manipulate some political system of this world.