Excuses were not lies per se. They were... creative reinterpretations of the truth – perhaps not the first thing one would think an angel capable of, between their apparent lack of creativity and rigid adherence to certain rules of engagement, first and foremost being the necessity of capturing the moral high ground, but there were occasions when it was less than beneficial for a recipient of the good word to know the whole story.
Using the same techniques on a fellow warrior left a sour taste in the mouth. Using them on a brother was unthinkable, and yet use them Castiel did. Perhaps he really was falling. Perhaps he ought to have prostrated himself and invited punishment ('begging for forgiveness' did not suit creatures who were supposed to be sure and certain and unflinching).
“Am I to assume the mission has changed, then?” Castiel tilted his head slightly, trying not to flinch under Zachariah's scrutiny; ironically, maybe, it was made easier by remembering what the garrison (he did not remember who specifically – maybe it was Anna. He hoped not) had taught him about facing demons - remember everything that's bad about them and you won't flinch. “You wish me to influence Dean Winchester. Dean trusts Samuel, Samuel trusts the creature 'Ruby'. Neither of them trust us. My best chance – our best chance – is my convincing them I am not like you. This... pretence of friendship? It is to that end. To smite her now would only show that I am 'just another douchebag angel', would it not?"
It made sense, as a tactic. Distinctly un-angelic in flavour though it may have been.