Can a Validator in a PoS System Execute a Successful Double-Spend without a Majority Stake?

No, a single validator cannot successfully execute a double-spend. While they can propose a block that contains a conflicting transaction, the network's consensus mechanism requires a supermajority of validators to attest to and finalize a block.

Without the majority stake, their malicious block will be rejected, and they risk being slashed for proposing an invalid or conflicting block.