What kills us in the event of two galaxies combining? I find the joining of galaxies interesting as the two eventually become ONE. On series I saw on a discovery channel stated we die.  Everyone dies. What is it that causes us to die when the galaxies combine? Is it something colliding with the Earth from the other galaxy such as a planet? Is it the drastic element changes on the Earth such as extreme temperatures or flooding, etc. Do we have any percent of surviving on Earth?
There are two reasons why nobody will die when the Milky Way merges with Andromeda:


The merger won't take place for another 4.5 billion years, by which time solar evolution will have long since made Earth uninhabitable (and even if humans did colonize the outer planets, we'd have long since gone extinct or evolved beyond recognition).
The merger itself will be a slow and not very dramatic process - stars aren't going to collide, planets aren't going to be ejected from their solar systems, and while it will look dramatic from a distant galaxy, other than Andromeda looking larger in the night sky not much will change for any intelligent life that's around at the time.  


However, there's one potential wrinkle. Both the Milky Way and Andromeda host supermassive black holes at their cores. We've never observed a merger between two supermassive black holes, and modeling the process is theoretically challenging (look up the "final parsec problem" if you're interested). When this does happen, though - long after the merger of the stellar portions of the galaxies - there would be a massive release of energy. Most of this will be in the form of gravitational waves, but if enough of it comes as high-energy radiation it could be bad news for any remaining life.