You can't say that the student is the one responsible, as s/he is not in control, nor do they have the knowledge to know what is required.
I believe nature is a great equalizer. We are all equally responsible to carry our own weight or suffer the consequences of not doing so. This may sound harsh or callous, but if a student is not capable of recognizing that something is not quite right his skills or training -- in spite of what the C-Card says -- he will have a higher probability of winning the Darwin award. These are not my rules, it's natural selection.
Now, I fully agree with what seaducer says about not agencies not spitting out "accidents waiting to happen", simply because the numbers don't point out in that direction. But he brings a very valid point in that we are polluting the oceans with less than capable divers and this does have an environmental impact. Also, neither do I think scuba is as safe or safer than bowling, unless I've been watching the wrong kind of bowling all this time.
Let me try to illustrate with a personal experience: I got through basic nitrox. At that point I had very little knowledge of mixed gases because training designed in the 60's did not contemplate them. I was taught that %CNS is cumulative throughout repetitive and that I was not to factor in any credits for surface intervals. But then I note that, my computer does give me some credit for surface interval. So on my own and without intervention of an instructor I start doing research about it. It turns out that there is a half-life to accumulated O
2. It just so turns out that in the course of my quest to find the answer, I also find out about other things not taught in the course. For instance the time-dose relationship of exposure to higher ppO
2. Now I find that I will not certainly die if go to a 1.61 ppO
2 for one minute. I learn that in fact it could be more risky to spend 40 minutes at 1.4 ppO
2 than 5 mins at 1.6. I get a little annoyed that basic bits of easy information like this has been kept away from me. This further heightens my awareness that if I want to be truly proficient I need to go beyond formal education.
By the time I get to take the Adv Nitrox course, I already know how to use the NOAA oxygen exposure table and like it better than the adaptation the agency did for the basic nitrox course.
I think that if you keep your eyes and mind open and you commit yourself to scuba for more than just diving during your yearly vacation, anybody should be able to recognize their own deficiencies and should strive to alleviate them. If they are unable to recognize it, then we have a candidate for the Darwin award. If they recognize it and don't alleviate them once they are aware, then they should be the very first ones to get the blame, not the instructor.