The reveal of the Camerlengo as a villain seems, initially, to be an Ass Pull to induce Ending Fatigue. Mr. Gray's words, "these are men of God", which foreshadowed the twist, however.
It also references to the shadow council from the previous movie. He's likely worked for the Vatican before.
A smart viewer will realize from the Camerlengo's lines how full of shit he is, especially in his speech to the cardinals, when he argues that science should heed to religion, and this AFTER he tells Langdon how La Purga (the public branding and killing of 4 Illuminati scientists by the Vatican) is a "black spot on the Vatican's history". He basically negates everything he claimed before on that speech. Also, easily overlooked, his joke to Langdon how he looks good on a priest's clothes, after Langdon changed his bloodied clothes for a priest's shirt, while removing the white band. His comments make it clear he would like to have Langdon become the Church's subordinate, which has precendents (i.e. the Vatican forcing Renaissance artists and scientists to work for them).
Why not give the cardinals someone to protect them? Granted, it was an internal conspiracy, but still?
Church Law does not allow anyone but the Cardinals in the Sistine Chapel during the election; this is why the Camerlengo entering, even to try to save their lives, was such a big breach.
