Let’s be honest you’ve just described Rick Sanchez’ life arc!Where evil is the selfish me-first sort of evil.
So, set entirely aside the problem of backstabbing the party. I never do that - that's not fun (unless that's the point of the entire campaign), and I have never and will never turn mid-fight or steal resources or anything like that. D&D is a group game, and I as a player fully recognize that and have and will always abide by it.
Ok, that aside - whenever I play a pure wizard, the higher they get in levels the more capital-E Evil they become (although never chaotic stupid or extreme murder hobo, but often very, very selfish). And, come to think of it this is really just wizards, never when I have played a warlock or sorcerer, although warlocks can easily trend morally grey/neutral - and multiclassed or dual classed characters never really have the same sort of trajectory. At some point it just becomes blue and orange morality, but it goes through me-first "evil" before it gets there.
It starts, often, with a bit of necromancy - because of course it does. Then come the fiends, and before too long its all about extending life and acquiring more power to extend that life - vampirism, the unholy lich-grail, deals with ruinous powers, etc. More spells, more spell books, more powerful items. Once you can hop the planes, the local matters on whichever planet you happened to hail from seem trivial. Oh? That city will be wiped away? Oh well, can't win em all. Maybe you'll help the party out because it feels a little bit like a family/class reunion of sorts, but then it's back to planewalking. Some of it is how much Jack Vance is still in the wizard spell list - even today, and some of it is just old tropes; hero to Raistlin in just 15 or so levels, right? Along the way they stop agreeing (if they ever agreed) to do things for free, and of course "for the experience" isn't really a motivator with much verisimilitude.
Because of how wizards acquire their powers, this seems like the natural progression for most of them, and the more power they acquire the less the small things seem to matter for them. Other classes don't seem to progress in quite the same way, and I am having trouble putting my finger on why. It's been a long time since I have played a sorcerer, but being so good at the social game makes them more embedded in social structures to my mind - plus the spells known problem makes some of the more bizarre stuff wizards can do too costly in terms of resources. Warlocks are flat out constrained by their list, as are Bards to a certain extent; although a particular kind of murderclowngod bard could very much go down this route, too.
This is such a common turn of events I try to plan against it with backstories and so on, but it never lasts. Some time around levels 9 or 11, they start turning to the dark side. And, again, this is really only wizards. Warlocks? No problem at all. I'm starting to think I should set a "become evil NPC by" date for them when I make them up.
Three things that alter that curve.
- A family
Find something that matters more to you than power. Either something you’ve lost that mattered to you (like your surgeons hands for instance) or something that you need to protect (like a grandson).
Incidentally there are plenty of ways of extending life aside from drinking blood or drinking poison and secreting your soul in a box.