Can I change the code of the node and runit usually

Question

Lets take ethereum for instance(this question is general for all the public blockchains). Can i change the code (like sending transactions but not deducing account balance) and just run it as usual. Is there any underlying mechanism that prevents this? I mean how is the system secure if i could run a modified version of it?

Answer 1

Say, you run an unmodified ethereum node, and you get 3 ethers from somewhere in your account. Now you rebuild your own modified node so that you send 2 ethers to me, but does not reduce 2 from your balance. What happens is that me and the rest of the world will verify and process the transaction that you signed and reduce 2 ethers from your balance. We don't run the modified version, right? You see what's going on? The next time you try to spend another 2 ethers, your node will think you still have 3 ethers and will sign and broadcast a transaction, but everyone else in the world will not accept your transaction, since they know you only have 1 ether. So you harm just to yourself.