Two things:

1. You said "we know that when the brain dies, our mind die, and is gone forever". In fact, we do NOT know this. That is a belief, or an opinion, not a point of fact.

2. Karma, in my opinion, basically means "what goes around, comes around", or "as you sow, so shall you reap". Walking on an old sidewalk is not karma. Its being there has nothing to do with my actions, good or bad.

Karma in the traditional senwe would only work in conjunction with reincarnation. In the new age sense, it's just a flaky, feel-good belief with no justification. Karma as you present it is inheritance.

That just brings me to there's no such thing as karma.

Your new notion of karma seems awfully similar to cause & effect.

Wouldn't you need supernatural being to effect such karma?

I've seen a number of stories about peoples' lives that were ruined when they won the lottery. Unlucky bastards, huh? I know it's wrong to read those headlines and just laugh, so I read them carefully and try to empathize on what it must've been like to live with next to no money, in some run down trailer, eating nothing but processed foods, and splurging to buy a couple lotto tickets, and then one day, BAM!! You've got the golden ticket and you're movin' on up, to the East side, to a deluxe apartment in the sky. You've got cousins 9 times removed asking for money, your best friend's 30 year old truck broken down and it'll cost 40 grand to replace it, and your neighbor's dog needs some super expensive life-saving surgery or it could die in a matter of days. Before you know it, you've given away a massive chunk of your newfound fortune, and then the IRS stops by to say hello. I think about all those what-ifs and how that poor schmuck's life was ruined when he won the lottery. And I laugh my ass off. Lottery didn't ruin his life. It was chum and he found out how many sharks he was swimming with. That the kind of karma you're talking about?