Violence takes all that complexity and adds more. Some of the complexity is at this level. The person may be trained or untrained, may value a strategy that you have never seen or rely on one that you ignore because you have been taught that "No one really attacks that way."
There is more at other levels- social interaction. Motivation. (Almost nothing you have learned in sparring applies to a predatory assault). On and on. It is complex in a number of measurable ways- over time and technique and environment; and fuzzier ways- the emotional baggage on all sides; and then the interaction between each and every piece.
That's preamble, because I'm going to ignore most of that, just didn't want anyone forgetting it.
I originally divide these aspects up by time. The idea was that the first stages are things that you need to understand and apply well before anything bad happens, the rest are in the order you will face them. This is my opinion, but if what you are training doesn't cover these seven things calling it self-defense is wrong.
1) Legal and ethical issues. If you do not know when it is legal to use force or what levels of force are appropriate or how to justify force you are vulnerable. I have seen people teaching throat chops to a hand grab... and people teaching ineffective joint locks for situations where deadly force would be not only legal but prudent. You don't want to be beaten, robbed or murdered- but you also don't want to spend years training to put yourself in a state prison. Trust me on that. If you don't absorb Force law with force techniques, you'd better be playing a video game. There are a bunch of kool-aid drinking reactions to this, most centered on a subconscious belief that the instructor is the next thing to god so if he didn't teach it, it wasn't important. It's far more likely that what he is teaching arose in a time and place when the rule of law wasn't even imagined. It is now, and ignoring that change in environment is as stupid and self-defeating as practicing archery from horseback and pretending it is for modern war.
Ethical issues are more personal. There are things that a normal person simply can't do, but they vary for each individual. And there are things that a person can do in an instant that they can't make themselves do if they have time to think about it. And there are things that people can do and can't live with. I don't know where these lines are for you. Neither do you. You know some of them and you suspect many more- a lot of the tough guy blustering is hiding these fault lines (places you might break) from yourself and others*. There is an art to training for this and a good instructor can notice your glitches and start working some of them into the light of day. The kool-aid drinker path with this is, "I'll be able to do it when the time comes." This is pure talisman thinking. If you can't make yourself do it now, you likely won't even think of it when you are surprised and scared. Work out your ethical issues with violence NOW. If you find that you have hard limits, don't waste time training to do things that you can't do or can't live with**. That requires understanding the effects of what you train. I suspect that many people practice a technique and never absorb that it is not just a hand or foot moving through the air but is intended to make a man blind or crippled.
2) Understand violence dynamics. You need to know how fights start and how predators attack, and those are two very different things. If you can't see it coming, you can't prevent it. If you can't tell, early, the difference between someone starting a Monkey Dance, setting the stage for a Group Monkey Dance or a Charm Predator closing to range, it is a crap shoot. The body language that prevents one triggers the other. The tactics that are necessary for a predator blitz can lead to criminal charges in a MD; a MD response to a blitz can leave you dead or crippled. Neither works that well in a GMD. That's big picture stuff. How do bad things happen in your particular area? It turns out that where I am right now, they don't use knives at all like I am used to. Know the dynamics, know criminal tactics. If the instruction doesn't accurately cover these you are rehearsing for your instructor's fantasy life.
There is more here, it is a study in itself, and it interacts with you. If the most violent gang in your area ranges in age from nine to thirteen, does that change the way you think about responding? Why? How? If the threat is clearly out of it, say lashing out in a diabetic crisis, would you handle it differently? How? Why? Would you even know?
*Once upon a time I was walking into a room with one of the toughest-talking guys I ever met when the electric panel blew out in a series of 'pops'. He turned and ran, thinking it might be gunfire. Had it been gunfire, I would have been alone to deal with it. Probably better in this instance.
**If you practice a blade or gun art find someone who raises livestock and offer to help butcher. If you can't force yourself to cut the throat of an animal or shoot one (to supply food- this isn't gratuitous violence but simply where food comes from) you will have a much, much harder time with a human being, both in the action and dealing with the aftermath.