Traumatic brain injuries and dementia disprove the existence of a soul.
My definition of a soul is an immaterial entity, separate from our physical bodies, that will be granted a place in the afterlife (Heaven, Hell, purgatory, or any other immaterial realm that our physical bodies cannot access, or transferred into another entity to be "reborn"). The key part of this is that the soul is "immaterial", meaning that physical occurrences do not impact the soul. For example, death does not damage the soul, because the soul is "immortal" and when the physical body dies, the soul is transferred into another form (whether this other form is an afterlife or a rebirth or anything else is irrelevant). We can call this the "immateriality" requirement.
The other requirement for a soul is that it is a repository of who you are. This can include your memories, personality, emotional regulation, or if you have anything else you think should have been included please feel free to comment. I will summarize these traits into the "personality" requirement.
So this brings us to the concept of brain damage. Brain damage is when you incur an injury that damages your brain. Depending on where this injury is located, you can lose your emotions, memories, personality, or any combination thereof. The classic case is the case of Phineas Gage. However, Gage was hardly the first or only person to experience this, you can find many others.
If the soul is an immaterial repository of your personality, then why is it able to be damaged by something material like brain damage? Brain damage is not the only way either--tumors, drugs, alcohol, electricity, oxygen deprivation and even normal aging can also damage your brain and alter your personality.
If the soul is not immaterial, then why is it able to survive death? Why is a minor damage able to damage your personality, but not a huge damage like the entire organ decomposing?
If the soul does not involve your personality, then in what meaningful way is it "you"?
Traumatic brain injuries can cause memory loss, personality change and decreased cognitive functioning. This indicates the brain as the center of our consciousness and not a soul.
If a soul, a spirit animating the body, existed, it would continue its function regardless of damage to the brain. Instead we see a direct correspondence between the brain and most of the functions we think of as "us". Again this indicates a human machine with the brain as the cpu, not an invisible spirit
Besides the lack of any evidence for a soul, it's pretty easy to make a human appear soulless by damaging specific parts of brain matter. The brain drives consciousness and our experience of being human, once it dies so does the concept of us.