Exactly. A blockchain is supposed to provide a set of guarantees so the nodes don't have to trust each other. If there is already trust, why bother with a blockchain in the first place? If there isn't trust, then there's definitely a possibility that one of the untrusted parties could try and attack or disrupt and the shared ledger.

Trust is not always so black and white (as on the open internet where anonymous people try to scam each other), in a lot of business cases there are different levels of trust which needs to be handled in a different way.