I have been at that very trailhead and very trail more times than I can count. I have often taken out of town folks along the Pinnacle Lake trail since it is relatively short (two miles) and yet opens up to a very classic Cascades alpine lake set into a rock bowl. Lake Twenty-two, Mt. Dickerman, Perry Creek, the Big Four ice caves....I go to the Mountain Loop Highway a lot, since it is only a 40 minute drive from my house in good traffic.
Oddly enough, in my 40 years of hiking/backpacking/snowshoeing/xc skiing, all in the Pacific NW, I have never even had my car broken into at a trailhead. This is just a tragedy for my two fellow hikers. Since I, or no one else yet knows the circumstances of their death, it is premature to speculate if carrying would have been of any benefit in this situation. As an example, were they confronted up close, marched off the trail and shot, or were they shot from a distance and the bodies then posed?
I routinely carry in the woods, but if someone shoots at me from 100 yards away with a rifle, the first thing I will know about it may well be feeling myself fall to the ground from the impact. My SP-101 will be of little benefit to me in that scenario. In an attempt to stack the odds in my favor, however, I carry nonetheless.
There is some talk on the Pacific NW hiking forums of having a memorial hike on that very trail. If so, I will be there. I just won't let anyone see that I will be carrying.
PS: Edited to add: the PI article linked at the top of this thread now has 65 comments posted as of 23:03 hours on 7/14/06. Interestingly enough, the majority of the postings are on both sides of the carry issue.