The discussions about whether or not Nick was a POS before the drugs are purely academic. At this point he could have been a saint or satan, and he'd still end up the way he is now. Hard drugs specifically fuck up your brain and alter your personality to turn you into a piece of shit.
Lemme break it down:
1. They fuck up your brain's Dopamine system.
Your brain uses Dopamine to regulate emotions and reward you for good or productive behavior. It's why you get out of bed in the morning, why you work hard, try to achieve goals, etc. Lots of things can give and take away Dopamine, but hard drugs like cocaine actively rewire it. Your brain stops producing dopamine for anything other than drug use, and you develop less and less as your tolerance builds up.
You don't do anything productive, because drugs become the only thing that gives you dopamine. Thus, naturally, they become the only important thing.
2. They make you entitled.
As you can imagine, if you're always feeling good all the time, and all your negative feelings are drowned out, you're naturally going to feel special and important, and like you deserve or earned it. Because your brain chemicals are reacting the same way as if you had. Consequentially, the only time you ever feel bad is when someone gets in the way of your high, or tries to remind you of a responsibility. You feel like you just won a trophy and everyone is trying to take it away from you. Imagine if you fell in love with someone and people wanted to pull you apart. That's how fucked up your brain becomes: you feel exactly like that's happening when people tell you to stop being a crackhead.
3. They make you retarded.
Your Pre-Frontal Cortex, Amygdala and Hippocampus are all stunted by hard drug use. These are the parts of your brain that control impulses and emotions, your rational decision making, your ability to contemplate long term consequences, your ability to remember important information, and your ability to concentrate. What kind of person lacks the ability to control those things? A child.
An infant can't regulate emotions, think about long term consequences or make decisions based on rational thought. Those parts of the brain start out fairly inert and become more functional as you grow older. It's what separates children from adults. When you shut these parts of your brain off or otherwise numb them, you lose all of those important skills you need to function as an adult. Therefore, you become mentally retarded.
Nick doesn't have a personality anymore. His personality is "Crackhead." They're all the same because the drug does the same shit to everyone. If he weren't using whippets as a crack substitute, he'd be going through painful, agonizing withdraw and his brain would be forced to rewire itself to function normally again. He's essentially now trapped in a state of self-induced retardation, and because of the whippets, his brain isn't healing.
TL;DR: Nick is a crackhead, and all crackheads are exactly the same until they get clean. He's not getting clean, so he's just a crackhead.
