>It was him the demon was seeking out, from the very beginning.

I have to disagree with this.

Father Karras was having a crisis of faith throughout the movie and was moving toward secularism. It took witnessing true demonic evil to restore his faith. If the devil is real, then so is God was his reasoning. The devil didn't destroy his faith. It re-affirmed it.

Kerras's final selfless act (taking the demon on himself) was spontaneous in nature, however. He had to invite the demon in before it could take him, just as it did with Regan when she was playing with the Ouija board early on. It would not have taken him otherwise.

If Pazuzu was pursuing anyone, it was Father Merrin since he first encountered the evil while on an archeological dig in Iraq at the beginning of the movie. He had also dealt with it before at another exorcism in the past.