Etherium explains how it can attack legitimate validators without a 51% stake

However, a strength of proof-of-stake over proof-of-work is that the community has flexibility in mounting a attack. For example, the dishonest validators could decide to keep building on the minority chain and ignore the legitimate fork while encouraging apps, exchanges, and pools to do the same. They could also decide to forcibly remove the honest player from the network and destroy their staked ether. These are strong economic options without the need for a 51% attack

https://ethereum.org/en/developers/docs/consensus-mechanisms/pos/#validators

Remember, any attack against a dishonest player works even better against an honest one.