Everyone with mental health illnesses and conditions are different in how their symptoms are manifested. It's individualized. It's not black and white. It's complex and complicated.
When you study the brain, and how the chemical imbalance, neurons, genes, hormones, and other factors such as trauma, childhood, environment, etc., interact in mental illnesses, then you will understand the behaviour.
Bipolar is a brain disease. How is it different than understanding when someone that gets erratic or violent with dementia for instance?
And yes, unfortunately some can become narcissists in a manic episode.
Mania in bipolar is an inflated sense of grandiosity and self esteem. For example, they can think they're the smartest people in the world or a genius or a God. You get an elevated mood, euphoria, you get elated, extreme energy, you're on top of the world and nothing can stop you. You also act out of poor judgment such as reckless and erratic behaviour, overspend and substance abuse. You can't control the mania and it's symptoms.
Delusions will make you believe and do things out of character. He maybe is naturally cocky or arrogant, so maybe if he has bipolar it only inflated the ever present traits, or it is the bipolar that makes these traits surface and act out.
It is no secret Kanye was taken into a mental health institution under 5150 involuntary hold and that is NOT just for any reason. That's because dude was seen as a danger to himself and has mental health issues.