I understand that if one node is compromised, it can then be used to generate malicious txs and the attacker can try to add them to a block it generates, but why would you say the other validators accept such block(s) or transactions in the first place?

It seems to me that a successful attack on a private blockchain requires the attacker gaining access to both a "pre-approved" node and having the hashrate majority needed to ensure its blocks are generated faster than the rest of the network combined.

Some background: https://coinjournal.net/vitalik-buterin-on-misconceptions-in... and https://www.reddit.com/r/ethereum/comments/48m9n6/vitalik_bu...