At one of the earlier startups I was involved with I had worked closely with another developer. When said startup fell apart and the group of us all went our different ways, I got a warning from another colleague about the individual I had grown close with, we will just note them as "E".
She had cautioned that when things were good (and the usually were) "E" was a great individual. However, the moment things did not go 100% perfectly, or there was any type of "advantage" to gain regardless of the cost, they would immediately do that and backstab anyone. I wrote that off as just a disagreement between the two of them, something grudge or who knows.
It took many years later that "E" made his true nature shown. I truly believe it to be a combination of being locked down for COVID, constant job hunting, the fact he had "quit" learning in tech, a weird fascination with crypto currency, getting lost into a hole of IDPOL (us versus them, with us or against us nonsense), and some closet fascism to boot. Not to mention attacking myself and others for "being educated" because the few of us Engineers held specialized degrees and long work histories, whereas "E" was nothing more than a basic bootcamp grad.
All it took was myself not fully agreeing on a trivial point in a matter, and in moments "E" had attempted to turn everything I had privately confided in him in an attempt to hurt me. Of course all that did was confirm the warning of him being what he was, and his opportunist ways.
I disagree. I'd say that you don't know what kind of person you have until they've put in a few months of work.