So if any individual node can create a public and private key pair, that particular node should know my private key, right? And there is no way to change the private key.
If a bank gave its customer the PIN for debit card and told he/her that he/she could not change it, everyone would fear for the security of the money in that account, right?
I’m not sure what you’re getting at here. Any node can, and indeed does, create their own key pairs, but they must always keep their private keys secret. The number of available key combinations is such that for all practical purposes, private keys are unguessable. So you’re the only one who should know your private key.
As for your card example: FWIW, the credit and debit cards I’ve used have usually come with a preset PIN that I have not had any means to change. But payment cards are different from bitcoins in that you also need the actual card; you cannot make purchases from my account by knowing just my PIN. Additionally, if someone at my bank would abuse their knowledge of my PIN code, they would stand a good chance of being caught, and the transactions would probably be reversed in any case if I could prove the charges to be fraudulent.
Bitcoin transactions, however, are anonymous and non-repudiable. Anyone who knows your private key can instantly transfer your bitcoins to their address, and investigators can’t easily ‘follow the money’. This means that if your private key is compromised, you’re likely to lose all balances associated with that key, and you will have very slim hopes of ever catching the perpetrator. There is also no central authority that would be able to nullify fraudulent bitcoin transactions.
