I'm not quite sure if you meant it as an example of ineffective advice or not, but the "be wary of snakes" advice is quite close to what I give my mentees, although tailored to my predictably Munger bent of "be wary that other people's incentives often don't align with yours." (As a note, I think we're using the word "snakes" for purpose of context consistency, it's not a word I'd usually use when discussion the actual behaviors here since it paints the conversation a very specific way)
I think perhaps the issue here is that I'm coming at the discussion from over-correcting the opposite direction; I was raised with very much a "people are All Good(TM)" outlook, regularly being fed the "Things get better after highschool/people grow up" line as a child. I've found that these both gave me ineffective priors for navigating adult life, largely due to that if you assume people are acting with your best interests at heart, you may be less inclined to look out for your own best interests yourself, or at the very least how other people may be compromising it, consciously or not.
Framing it in terms of incentives also has the wonderful side effect of circumventing a lot of they "are they malicious or just self-serving" question since it hops right to "how do you fit your success into whatever equation everyone else is trying to route into"; my inquiry of whether they were TRULY malicious is usually made post-factum (or given sufficient evidence)
As a final statement (and to couch my own incompetence with tone over the internet, please don't let any of this come across as a You're Wrong!), I'm not sure I'd agree with your statement that people with bitterness outnumber those who would manipulate others, at least in my work context. In the lense of my "incentives" stance, both are continuums of behavior, but the bitterness continuum is counter-productive to career advancement while the "manipulating incentives" continuum is productive, so I'd logically expect to see more of the latter. (I guess the real crux here is "when does it become malicious," but that's a whole separate discussion)