I suspect if you asked 5 of the most chashuve rabbonim, you will get 6 answers all of them highly nuanced and caveated . The reason is simple: The term “mass shooting”, “self-defense”, “stand-your ground”. and the underlying facts and context are so varied. All will agree that the classical notion of rechitza is assur whether one victim or many; jew or non-jew . All will also agree that individuals are entitled to take reasonable step to protect themselves and their families. Beyond that there is unlikely to be agreement on a single hashkafah any more so among rabbonim as among members of Congress seeking to find a common and unifying theme on issues such as gun control, etc. There is also a question of how a yid living in galus must act under dina d’malchusah versus halacha.