What Are the Key Motivations for a Selfish Miner versus a 51% Attacker?

A selfish miner's motivation is purely economic: to increase their share of the block rewards and transaction fees beyond what their proportional hashrate dictates. A 51% attacker's motivation is broader and often more malicious, including the ability to censor transactions, perform double-spends to steal funds, or even destabilize the network for ideological or political reasons.

The selfish miner seeks profit; the 51% attacker seeks control and manipulation.