While the pacing has been a bit off, I think we've seen for quite a while now that Ten has been going off the rails, and that RTD has even intended for it to have consequences. Every time Ten has pulled less extreme versions of this shit, it's eventually come back to bite him in the ass - his attitude toward Queen Victoria rather heavily foreshadowed Rose's loss, and by deposing Harriet Jones, he opened the door for the Master. As you pointed out, "The Fires of Pompeii" was the moment at which he finally OFFICIALLY ceded the role of conscience over to his companion (Rose was too fawningly approving, and even Martha a bit too in love with him to call him on his bullshit as much as he needed, but Donna possessed Sarah Jane's gift for forcing him to level with her).

Ten was frighteningly like Four here - later-phase Four, that is, after "Genesis of the Daleks," when he seemed ready to reverse his position in that episode. This is Ten as Four in "The Invasion of Time."