It starts off with a small lie to yourself - I’ll just use this money to pay him and when the investment pays off it’ll all work out... and then it grows from there.
Cognitive dissonance is an amazing quality of the human brain. I recommend reading Mistakes Were Made But Not By Me - fascinating stories of how and why it happens.
I've known people with "get rich schemes" who ended up lying and stealing from their loved ones.
But the self-serving excuse of "I'm only human" is always a lie. That's not being human; that's being a particular, untrustworthy kind of human. Not all of us, or even the majority, are like that. "But it's just my nature" is such a cowardly excuse for lying and cheating.
I'd like to think I haven't really done anything like that since I was a teenager, but as you said, cognitive dissonance is amazing. It's entirely possible that I do things all the time that are horrible.
I seriously doubt I'm a saint; I think most people don't want to hurt other people.
(If anyone is interested in the conclusion of that story, I was able to eventually find a job and after a million calls to the landlord I got him to let me pay a bit later and keep my apartment).