As I pointed out previously, likely they were killed to prevent being rescued by the IDF. As seen on Al Jazeera, it seems that the people supervising the hostages have been given orders concerning what to do in the event a rescue is likely.
Looks like the IDF have failed in both their objectives, to eliminate Hamas and rescue the hostages, and the only way to get any more out alive is to have a ceasefire and hostage prisoner exchange.