The brain is physical.
So, my opinion is our brains do not decide how we are in our personalities. Biochemistry, then, does not determine our personality and emotional makeup.
Then why can brain damage cause people to have different personalities?
My short answer is > it looks that way. But the change is really only outward. How the person really is, deeper, has not really changed. But after brain damage, now the person's physical world is different, and how that person is deeply can come out in reaction to that.
Now, if you please to go through much more detail about this >
I think you already know how any of us can go through changes between being nice and nasty, depending on how things are going for us. So, if someone's brain gets changed, this can effect if the person is getting what he or she wants, and the person can be reacting somehow to how he or she perceives things to be going for or against the person. And that perception, without Jesus, can make things seem pretty bad for the person. And my opinion is our perception comes from deeper than our brains. Any of us can fool our own selves, if we are not trusting how God guides us and our perception and emotions in His peace. There are people who have not invested in making sure with God, about things and how to see things.
What I understand is my personality is not only what shows. It is how I am, deeper than my physical brain. And this includes my spiritual character which has a lot to do with how I react to things.
If I have the character of love, I am not going to react nastily and cruelly, even when things go against me. So, you could bash my brain, and it is damaged, but my deeper character will win out.
"Love does no harm to a neighbor." (in Romans 13:10)
So . . . for me . . . I understand I need to trust God to make sure I am deeply His way in His love so I do not become nasty and violent and dangerous after being brain damaged or something else physical which comes against me. You can read and feed on how Paul became because of God's grace changing his personality > 2 Corinthians 12:7-15, Philippians 4:11-13.
But this is talking about what is possible with God.
With humans, it is very possible for a selfish person to get bashed, then forget how to act in a civil way. With things so changed by the brain damage, the person might not be so interested in acting nice, so he or she is more ready to get angry and cruel, even; but my opinion is that nasty reacting is more likely if the person is not getting his or her own way . . . somehow. And the person is likely being controlled a lot; so this alone can aggravate the personality of a person who has been one's own person.
So, I see how it is possible that there are people who have behaved very nicely, but they have also maintained their deeper ability to get violent when things go a certain way. So, that ability can come out after their brains have been damaged and they have lost control.
But it can go another way. There are caring people who can get very vocally violent when they see things go wrong for their causes. But after they get bashed, they do not remember all that which was their cause for anger. And then is when they can seem quite tranquil.
But that is conditional.
In God's love we have almighty peace to keep our hearts and minds, no matter what > Philippians 4:6-7. Humanly we can have our own free will's control as an idol, and in the weakness of this we can give in to various mental and emotional chaos and torment, while perceiving in a very unreliable way so we can more readily get troubled and wreck how we are relating; so we need how God is able to cure our personality so we are submissive to our Father in His peace with almighty power to keep us this way > Colossians 3:15, Philippians 4:6-7, 1 Timothy 2:1-4.
