I once heard an attack against LN, which was called "flood & loot": https://medium.com/blockchains-huji/flood-loot-a-systemic-attack-on-the-lightning-network-5c3dac7bba24
The 3rd step of that attack was described as follows:
III. Resolving payments on the last hop
After all payments were successfully routed and the HTLCs were added to the target node’s channels, the target node resolves all payments by returning the required secrets and claims these funds for himself. At that point, the target node could gracefully close his channels and leave with the funds sent by the source node. Once each victim acquires the secrets, he sends them back to the source node, asking to resolve the HTLCs and move their amount to the victim’s side of the channel. The source node refuses to resolve the payments and ignores any further messages from his victims.
So what does "resolve the HTLC" here mean? I see a mechanism titled "HTLC Off-chain Termination" in the LN whitepaper - does the term "resolving" here refer to this mechanism?
If so, I then think that, resolving/terminating HTLCs of the target node off-chain gives the attacker advantage (or gives the victim disadvantage) when the HTLCs of the source node is not yet resolved/terminated off-chain. Because, once the HTLCs of the target node are resolved, the victim will completely rely on the HTLCs of the source node being executed (rather than expired/refunded) to merely avoid fund loss, at best, meanwhile, the attacker will profit by stealing fund from HTLC expiry/refund, even in the worst case the attacker still won't lose anything.
So, what if the victim node in the middle then refuses to resolve the HTLCs with the target node as well, as long as the source node refuses to resolve its HTLCs with the victim node in the first place?
Why, how, and when should an intermediate LN node terminate/resolve its HTLC(s) off-chain?