Curious to know if you think shooting down United 93 on 9/11 would have been the right thing to do or not? (had the US Airforce got there in time). What say you?

Will the criminals take more hostage if we don't act? Are the hostage takers willing to return the hostages safely? Is it feasible to attempt a rescue operation that doesn't just put many more at risk? Does this criminal organization's continued existence have a human life cost to it?

Not the same thing. The hostage takers in your example are only posing a danger to the hostages themselves. There is no outside harm caused by deciding not to act. Hamas on the other hand poses a real danger to Israel. Not counter attacking Hamas would result in attacks on Israeli citizens, so Israel has to act. Hamas is using the hospital as an unethical way of shielding itself from such counter attacks because they know Western war philosophy aims to minimize civilian casualties, so attacks on hospitals hurt Israels global support. However, by operating from within a hospital, Hamas are making the hospital a valid military target. Minimizing civilian casualties goes both ways. You don't attack hospitals unless they are a valid military target and you don't set up military operations inside hospitals so as to prevent them from becoming valid military targets.