My guess is that it gets to the first Wait, and waits however long, then it starts monster flashing and initiates the second Wait. So if monster is destroyed, that second Wait is still being processed. When the second Wait continues it doesn't spawn any (am I correct?), and then tried to do the other events but picks all other bullets instead.
You could try using the Timer behavior instead of Wait. The Timer will allow each monster to have their own timer, and if the monster is destroyed, it's timer won't fire.