I'm new to aikido (having done some judo several years ago) and can truthfully say that at the age of 31 I've never felt threatened in any way even slightly resembling the scenario described. I am a man of peace and I am taking up aikido because it is a way of peace and harmony which allows me to develop physicality. If I wanted to learn to reflexively kill or maim people I'd have taken up jiu-jutsu.
I have observed in the past that a lot of people who have done martial arts seem to unconsciously draw violent situations into their sphere - either out of a need to prove their prowess or to be taught lessons of their own.
If one has done this and a knife-wielding uke presents themselves for education - educate them to the best of your ability. If this means you have to break their arm or dislocate their shoulder to disable the threat they present then *shrugs* they chose the education. However, if you are trained to the point where you don't need these things and could happily weave around the knife without being hurt and manage to turn your opponent so you can simply back out of the alleyway, there is no real threat. With no true threat there is no longer a need to harm uke - no matter how drug-addled they are. The education process is no longer on the risks but on the futility of violence. To teach a lesser lesson when capable of this one is a gross ethical breach and a sign of a lazy mind.
We are all here to learn, and to teach what we know. To teach a lesser lesson out of pettiness ("how dare they pull a knife!") or ego ("I must dispense justice!") is to fail in your responsibilities to yourself and your society.
I hope that makes sense. I'm new to the physical aspect of aikido but the philosophical and spiritual aspects of the art are very much in keeping with how I believe life should be lived and their concepts are old friends.
Harmony to all