Probability and statistics.
For example, if you are interested in a particular major, what sort of employment prospects can you reasonably expect from it? Can you afford the school you want to go to, and if not, what sort of student loan debt are you looking at and will you be able to pay it off with your desired major? How many unemployed or under-employed graduates are there who got an unremunerative major from a school they couldn't afford, or worse, went to graduate school in that subject with the hope of getting a good adjunct appointment and ultimately winning the ever-dwindling tenure lottery that are now unemployed or underemployed and saddled with student loan debt they can't pay off?
These people never bothered to run the numbers and had at best only a vague understanding of what might be in store for them but convinced themselves that they were special, that they would be among the elect who "made it," regardless of how stacked the odds were against them.
Study cognitive biases and fallacies and use them to examine your thought patterns and actions. Cognitive dissonance is, in my opinion, the most important, as it allows people to make mistakes without recognizing them as such and thus to make them again and again. Every time you make a decision that has unexpected consequences, even if they are favorable, ask yourself honestly, was this really a good decision? What evidence did you have at the time to justify it? Would you make it again, and if so, why? Would other people agree with your reasoning?
People can go through life blissfully unaware of how catastrophically bad some of their past decisions--for example, of where to work, where to live, who to marry--have been because of this all-power cognitive bias. Your goal is not to obsess over the past (and there is a risk of that), but rather to learn from it, and cognitive dissonance impedes you from doing that.
By "not fully responsible" I was trying to sidestep a free will debate. My point was that "bad" people might just have "bad" brains; perhaps they were exposed to too much serotonin while in the womb or inherited a bad set of genes, and that plus some trauma early in life might have damaged them in such a way that they were willing to commit unspeakable acts that "normal" people would not. I think it's not unlikely that whatever makes a serial killer a serial killer will eventually be identified, screened for and cured. But what to do with existing serial killers is different problem.