You will also note that Pacelli was not the only person to ever have balanced one evil against another.   Note Churchill's famous quote the night before Operation Barbarossa started:

"If Hitler invaded Hell, I would at least make a favorable reference to the Devil in the House of Commons."

Nor was Pacelli the only person to think he could negotiate with Hitler and come out of it with a worthwhile outcome:   consider, Munich, Chamberlain and "peace in our time."

In fact Pacelli was one of the first to try to get a quid pro quo from Hitler.   In this case his goal was to assure that Catholicism would not be attacked in Germany.

However German Catholics, with a much clearer picture of what was actually going on in Germany, opposed the idea of a Concordat--and remained a strong party in Germany.

Cornwell p 133:

In the March 1993 elections "....the Catholic Center" (party) "which had conducted a courageous campaign in the face of widespread Nazi intimidation, remained impressively solid at 13.9 percent, actually gaining three more Reichstag seats."

Right up until March 1933,, then, German Catholicism, with its 23 million faithful, still comprised an impressive, independent democratic constituency that, together with the Catholic hierarchy, remained steadfast in its condemnation of National Socialism.   While the Center Party had no viable allies to form a coalition, and therefore no purchase on power, Hitler feared a reaction from the bastion of political Catholicism as a whole, a group that was naturally much larger than the Center Party vote, with extensive links and associations on many levels throughout the country. Because of his long-standing determination to avoid a new Kulturkampf and the attendant risk of a successful Catholic noncooperation or resistance, Hitler was not inclined to tackle the bishops head-on.   
Something nevertheless had to be done to neutralize them and it was here that Pacelli's Reich Concordat ambitions came to Hitler's aid.."

From Hitler's point of view, the ideal solution to the Catholic threat was precisely a summit agreement with the Vatican in all respects similar to the Lateran Treaty, which had outlawed Catholic political action in Italy and effectively integrated the Church into Fascist Italy."

"There could be no Reich Concordat, however, without the bishops reversing their denunciation of National Socialism".

more later