I don't either. It's a very odd character arc in general. I don't for a moment believe that Magneto would actually try to live a "normal life" and hide his mutant abilities after DoFP. Assuming that he does though, after all the stuff we've seen him do in First Class and DoFP, is it really believable that if he even wanted to save the guy in the factory at that moment, that he would even have to reach his hand out or react in any way? He can control metal with just a thought! So ignoring all the dramatic contrivance to engineer the tragedy, the next thing that surprised me was when they mentioned that his wife was also found dead along with the daughter. So after killing all those people... did he kill his wife too? Why? Isn't she the reason why he wanted to settle down to begin with according to this narrative?
So he basically kills 20 people in a small town, most of whom are totally innocent of any wrongdoing other than being in the wrong place at the wrong time, kills his own wife, and then aids in worldwide destruction costing billions in damages and probably hundreds if not thousands of deaths. He basically became Apocalypse's World Engine, to take a Man of Steel example. And after all that, because he somehow helped a little in fighting off Apocalypse at the end, all's forgiven? Why would Xavier even let him within a hundred miles of the school. Letting him help rebuild the school as well? Trying to convince him to stay too? Simply gross.