^I'm speaking of the earlier scene, when Batman saw him melt and sink into the sewer grate and took a sample of the goop. In the next scene, he was in the Batcave saying that Mason would die if they couldn't find an antidote. At that point, he had no reason to believe Mason hadn't already died.
Sarcasm aside I do think its much better in theory than in practice. Katana is already nothing like the awesome, actual Katana from the pre Reboot DCU, so I'd hate to see a team of characters I enjoy get the Beware the Batman treatment.