I'll preface this with the most dismissive of suggestions towards the argument for the evil path, people can just headcanon the original chapter one, if they're arguing for it then they've played it. Next, if you need any form of justification its arguable that Nicki comes to Guy with the intention of cheating on her long time boyfriend, which shouldn't be that far off the argument being made for the gold digger justification. Sure, one could suggest he's an awful, manipulative, abusive boyfriend and so Guy is the natural better choice, but its just as possible Guy is just her switching abuser.
Neon's assertion is correct, there shouldn't be a required justification, a person who acts nice from a position of weakness won't always act nice from a position of strength. There's entire discourse on this in politics, in fiction, in religion, in history. Obviously with many of those we have the benefit of hindsight, but its not altogether that uncommon a meek or downtrodden character or person who suddenly comes into wealth or power abruptly switches into a callous or torturous one. Sometimes those "justifications" can be seen as innocuous from an outsiders perspective, or they happen way back in childhood where this story doesn't occur. There is some level of implication through Guys sister that his familial relationship is complicated. The first scene already displays a character who, on a whim, funnels all of his wealth into a shoddy crypto-currency while drunk off his ass at the behest of someone apparently charismatic. That's bad and impulsive decision making. It would also serve to occasionally create people who believe that society owes them something, and who then go out of their way to justify bad decisions they make to gain their perceived dues.
The main disconnect as far as I perceive it seems to be an argument of "there is no required justification" and "there is always an original justification". Which I don't think need to be taken as exclusive assertions, the "there is no required justification" argument isn't necessarily suggesting that someone doesn't have a personal justification, just that any justification they make for themselves doesn't truly justify the action taken. Basically, if someone is going to do something they internally perceive as bad they're going to do it, then they'll search for a justification after the fact, rolling it back until one satisfies or ending on "It's in my nature". And for the "there is always an original justification", an initial justification isn't always shown, though it can still exist, sometimes it might only be inferred. To use the aforementioned Watchmen comic book as an example, the Comedian might have an original justification but that's never given. The Comedian is just a person who quite often uses his perception as a good guy to indulge in his basal desires.