When was there time to rescue and deassimilate that crewmember, or any other for that matter? I must have missed that part.
Yes, you much have. For later in the movie Picard did go and make an attempt to rescue Data.
In the moment, there was no time to rescue each crewman who was being assimilated to a sickbay that may or may not have been under attack (remember that scene?) and have the Borg crap removed. How many were there? Who would go from deck to deck to find the crew and take them to be deassimilated? How many medical staff would that have taken, and how many security personnel would be needed to fight off the incoming Borg?
While a movie is playing you have time to go grab a box of Milk Duds, their self-deprecating name and remarkably mild flavor are the most apologetic of the boxed candies, but the characters in universe are fighting for their lives.
When it comes down to the nut cuttin', you do what you have to do for your crew. If that means making the decision to kill one who is being/has been assimilated by the Borg, you make the best decision you can at the time.
If that decision is colored by your own experience, so be it. You will live with the consequences of your actions.