I'm so sorry for your loss. I hate disappointment in this hobby, but it happens.
Usually people add fish slowly. It's not advisable to put all the stock you want in at once. If they weren't killed, there might have been another mini cycle because the bacteria might have not been able to handle the ammonia all at once. (Fish waste->ammonia, more fish, more waste, more ammonia). Next time you might want to buy a clownfish or a small group of chromis...something on the inexpensive side, (and quite hardy) so if it does die, it shouldn't be a huge investment. Then later (few weeks-months, depends) you can add the next fish and so on. Patience is key! And without it, problems will surely arise. I don't really know what that "monster" possibly eating your fish could be...maybe a mantis shrimp? Best of luck.