About mental diseases and kamma

A small incorrect assumption you have made is that brain somehow is storing kammic seeds or even memories or even responsible entirely for proclivities of a human. It's not entirely true. If you go through current alt-scientific literature, (just to give you an analogy), brain is more like a remote control, not the controller itself.

...such as a tumour, dementia or a lightning striking on someones head? What if those persons begin acting impulsively and heedlessly, even if before those misfortunes they had completely normal -or even wise- behavior? Will they rebirth in hell or something like that?

What happens after biological death is that those kammaic seeds which have deepest sankaras will determine the next rebirth.

If a mentally ill person for an example say commits a murder, it will not be a bad kamma on his part because as the Buddha repeatedly says kamma largely depends on 'intention'. A mentally challenged wont have a bad intention while killing. It will just be an accident on part of the victim. No hard feelings.

So they wont be rebirth in hell. Whatever kammic seeds at time of death will determine the next birth.

Consider it like a mathematical surface with 'intentions' as 'dimensions' and 'actions' as 'magnitude' of vector and the 'self' as a 'position vector'.

With no intentions, actions will not contribute to magnitude of the vector.

When the body dies, whatever resultant that vector has produced it will go fit in similar mathematical surfaces in the spaces of Consciousness.

Please do understand that every human has a physical brain and a mind. According to Buddhism the physical brain dies every time we end our current life. However, the mind continues its energy (Caring Khamma seeds) base on the good and bad added on to it in each life during your endless lifetimes. One main Khamma seed decide your next life. In the case of sudden death or death during an operation or a person in coma the person does not have consciousness to control the mind. During sudden death – No time to think and you will not know anything -Lightning strike Death during an unconcise situation – your Physical brain and mind is disconnected temporary that’s the only reason you can’t feel or remember anything happen during this time.

This type of death is somewhat dangerous to a person who practice Buddhism or live a good mindful life. On the other hand, a person who practice Buddhism correctly or live a good mindful face this type of situations rarely.

LOGIC When you are unconcise the mind is still active and however you have no control over it or no feelings as it’s disconnected from Physical brain. Therefore, one of the Main Khamma seeds form millions of Khamma seeds stored in your mind automatically gets activated without your consent (You have no control) and decides your next birth.

I knew someone who was diagnosed as psychotic for most of her adult life. Psychiatric medicines were semi-effective in her case (so the psychiatrist called her "one of the lucky ones"), so apparently the symptoms were brain-related -- even if etiology is too complicated i.e. physical and environmental.

A couple of comments based on that...

It took years before finding how to get medical treatment, because she was never "an imminent danger to self or others" which is the usual legal requirement for imposing medical treatment. I interpret that as evidence that her intentions toward others were invariably or mostly kindly, even when her thinking was very confused (disordered in form and content). To the extent that "kamma is intention", I think that counts for a lot.

She expected to be reborn; but she didn't say where nor for how long.

• MN 57 says something about being reborn as an animal -- given her great sympathy for ("mind of goodwill towards") animals, perhaps that (rebirth in the animal realm) is a possibility.
• Conversely her "sins", such as those ever were, were all too human -- perhaps that means a human rebirth.
• Perhaps she visited other realms (brahma- and otherwise) "in this life", experienced "paranormal entities" -- to that extent perhaps she developed some equanimity about future experiences.