user127796: What about if I (as a hacker) change the amount from 5 to 1 for any transaction and put signature with my (as a hacker) Private Key and mine that block again. Yes, I lose  1 coin, At the same time isn't it an attack to target the public key (target walled address)? Because target walled address lost coin from 5 to 1.
How is bitcoin preventing this? Democracy?

user208: If I understand the scenario you're describing, you are effectively constructing an independent, second transaction that also pays the receiver, but using your own funds rather than the original sender's, for a smaller amount.
The two transactions will not conflict, and both will go through. So not only will you have done nothing to prevent the original transaction, you'll have donated some of your own money to the recipient in addition to it.

127796: <p>What about if I (as a hacker) change the amount from 5 to 1 for any transaction and put signature with my (as a hacker) Private Key and mine that block again. Yes, I lose  1 coin, At the same time isn't it an attack to target the public key (target walled address)? Because target walled address lost coin from 5 to 1.
How is bitcoin preventing this? Democracy?</p>
 208: <p>If I understand the scenario you're describing, you are effectively constructing an independent, second transaction that also pays the receiver, but using your own funds rather than the original sender's, for a smaller amount.</p>
<p>The two transactions will not conflict, and both will go through. So not only will you have done nothing to prevent the original transaction, you'll have donated some of your own money to the recipient in addition to it.</p>
