I can't agree with the assertion that he "didn't exhibit any of the immense ferocity that he showed subsequently to the others" or that killing Mikio in a ferocious manner wasn't intended. From all descriptions I've read, it was a particularly ferocious attack.
If there is a slight difference in "level of ferocity", I would attribute it to likely the overall arc of violence: He starts by the (relatively) mild attack on Rei, silencing him so as to prevent an alert to his presence by the others. His next attack is where the "real" violence starts -where the violence becomes brutal, vicious, and bloody. The start is Mikio, followed by the other two. His adrenaline is rising during the attacks. I guess I'm looking at this as not a series of murders, but a murderous episode, with a start, peak, and finish. That's why I tend to think that the pulling out/emptying of drawers, and other destruction in the house is simply that once the adrenaline and physical energy gets going, it tends to taper. It wouldn't just stop with the last killing.
Anyway, just my two cents.