I have a question about how POS (ethereum casper) works. I am a little bit confused. So let's say User A decides to be a validator. So he has to have at least 1000Ether in his account to become a validator. When he does that, his 1000Eth becomes stake and is frozen. he can't use it anymore. Now, when new transactions happen in network, as we know it finally gets put into block and nodes make sure transactions are put into a new block as merkle trees. Now it's time to validate a block.
What does that mean to validate a block, if transactions are already validated by nodes? why does a block need to be validated? how does a validator's node validate new block? I've also read about bets. What is bets? If a validator is malicious, he loses his stake or bet or what does he lose? That's the things i am really interested in. I'd appreciate it.