I rewatched First Contact the other night, and after the assault on Deck 16 and retreat to Deck 15, it is abundantly clear that Picard was negligent. All he had to do was use transporters to beam any non-Borgified crewmembers off the deck and decompress the remainder into space. Not only were the lives of his crew at risk, but the entirety of the Federation (well, just the humanity part of it). At least at that point, judging by dialogue, the Borg hadn't invaded other decks yet, so it would have been easy to blow them out into space, minimizing Starfleet casualties.