It seems like a chemical imbalance in my brain can be the source of my "self-destructive tendencies" and depression since I was 9.
Would a simple blood test be enough? What about my level of testosterone?
I don't want to go to a therapist yet because it will significantly harm my chances in future jobs if they find something.
the chemical action of depression is as yet unknown. negative on any chemical test. depression is always diagnosed by expressed mental symptoms. however you seem to be wondering whether your depression is chemical vs something non chemical. it is not.
i recommend seeing a neuropsych. they take a more physically oriented approach towards depression and will prescribe you a monoamine oxidase inhibitor or serotonin reuptake inhibitor.
regarding therapy I would not worry about future employers. most countries have laws protecting patient privacy. however in many places in USA it may hinder your ability to by a firearm.