Must say I don't understand why people are so worked up over what he did. I wouldn't have done it but I don't see how it really matters.
From what I've seen the criticisms are 1) based on him representing American distance running poorly by going out too fast and blowing up. This is fairly ridiculous and assumes that the rest of the world's running community bases its opinions of American distance running on the one guy they saw on TV in 2012 running the Boston Marathon.
2) The Boston Marathon is a prestigious race that should be taken seriously at all times (from the Runners World comments). Like above, this one is fairly absurd because anybody in the race who didn't try to run their absolute best would be just as guilty. Even if you assume that what Glenn Randall did was more public and therefore more insulting the prestige of the race worse things have happened at Boston (cheating to be the first woman, banning women). Going out fast getting media attention and dying does not register as a a big issue, which is why none of the elites or race directors complain. Ultimately, I suspect this argument is from people who train very hard to try and come close to qualifying for Boston and as such have built it into something it isn't. This is also one of the few instances I can think of where one's running credentials matter.
3) He stole media attention from the other runners who deserved it more. This argument is just baseless speculation. While he did get some media attention is assumes that the rest of the elite runners suffered, or at least were truly upset, about not having the cameras on them for those first 6 miles. The lack of complaint from the leaders, and the absurdity of that assumption speaks is sufficient.
4) It was stupid for him because he did not try to run as fast as he could. I'd think one's racing goals should be up to them (saying that it is Boston and therefore should be taken seriously is addressed above). I know plenty of runners who will go out fast in a race and see how they do, tempo a race, jog then kick in the last bit, all of which lead to not perfect results. I do not see why other people should care how somebody decides to run a race or what their goal should be.
5) He looked stupid in his Tri suit and should have avoided the attention. This sort of argument only comes from LetsRun and is really specious. For a start I hardly think a singlet and short shorts is the definition of style and seems to imply that if you are going to run you should fit in and wear what everyone else does (unless you finish 4th at the Boston Marathon). As a start what you can wear in a race should not be contingent on how fast you are. Then there are people like Dave Waddle (hat) John Walker (long hair) Pre (mustache) and Wanjero (arm warmers) to name a few. But really the important question seems to be why somebody is so worked up over what somebody wears at a race then vents about it on LetsRun. At best it sounds like a overly obsessive high school track official.
Those are the big ones at least. If somebody can come up with a reason why it was wrong to deliberately take a race out too fast and then die is so personally repulsive I'd like to hear it.