I think he signed the Concordat because (1) it was the traditional method for the Church to guarantee the rights of its members in a country. I think the tradition started back in the 12th century; (2) he was a career diplomat, and that’s what career diplomats do — negotiate “understandings” and agreements. I think he knew what the Nazis were up to, even in the early 30s, and probably figured it was iffy, but decided it was better to do it than not. I wouldn’t say he acted out of fear, but out of custom and rational calculation. From what I have read, he didn’t seem to be a particularly fearful person even though he was personally in danger several times. Of course, WWII and related events were pretty frightening!