In my head I kind of simplified it - if I can reorder the blocks in my history I can "reverse" a transaction, like "erase" that I bought a lambo yesterday so today I have not only the lambo, but the money that was in my account before I bought the lambo, too. But maybe me trying to over simplify and missing the forest for the trees (this is very much not my domain).

My understanding is limited. But in addition to not making transaction "not happen". It is better to make new transaction for money. As the transaction would still be valid later and could be included later. Thus "double spend".