...*sigh*...

First, Raziel can de-activate the Soul Reaver anytime he wants. If you want proof, check any FAQ on Soul Reaver 2. Press the Circle button twice to either activate it or de-activate it at will. The only thing besides Raziel that can de-activate the Reaver is Moebius' staff.

Second, no the Reaver does not just start feeding on Raziel's life whenever the hell it wants to. The reason it turned on him at the end of SR2 is because he had just killed 6 PEOPLE WITH IT. He used it to kill 6 humans in a row, and because it had no other target to turn to, it went soul-crazy and began to devour Raziel. I will say this ONE MORE TIME: The Soul Reaver must be aroused in order to drain Raziel's life. The Soul Reaver becomes aroused by being used to kill enemies. If Raz just hangs out with the Reaver activated, it doesn't harm him. Raz would have to kill Sonic in order for the Reaver to go anywhere near the drain-point.