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He didn't "steal" anything.

So exploiting a bug isn't stealing or cheating? Oo
And he has a witness account. How is that acceptable?

Everyone has a witness account. As soon as you get a single vote (even from yourself) and wait long enough you will get to produce a block.

I dont and if I do, I have no idea how to use it :p

But I guess what you are saying is that its not hard to become a witness x)

Marky also told me he is witness nr. 212, so hardly a witness.

Bug or feature?
He did just that, what could be done.
Where is it written, that he couldn't bet in that way?

A software bug is an error, flaw, failure or fault in a computer program or system that causes it to produce an incorrect or unexpected result, or to behave in unintended ways.

Copied from wiki :)

Thats how I see it.
You might see it a different way :D