1. The media will cover the massacre relentlessly for 5-6 days, interviewing victims, bystanders and government officials. They will cover the vigils and dedicate the 24 hour news cycle to it for as long as people can stand it.
2. There will be vigils held around the country and Americans will write "pray for Las Vegas" on their Facebook walls.
3. Anti-gun activists will get time on news networks to argue their case, until the media determines the story is old and no one wants to hear people debate it anymore.
4. Pro-gun activists will stay quiet for a couple of days before putting forward their response -- which will be to have more guns to kill potential mass killers. They will accuse liberals of politicizing the tragedy and argue that "guns don't kill people, people kill people".
6. Right wing media figures will begin to create narratives that serve the pro-gun agenda (Stephen Paddock was a "lone wolf", he was mentally ill, we need more guns etc, etc). They will make enough noise to convince liberals that there is little point in trying to change gun laws in America given how extreme their position is. Either you are pro-Second Amendment, or you are a Muslim loving communist who wants to take away people's guns, etc, etc.
7. Politicians will make speeches promising action. Liberals will plead with Republicans to take the issue seriously, and propose regulation on the most dangerous weapons and ask for more stringent background checks for gun owners. Republicans will then demand school teachers, children and newborns have access to military grade weapons to fend off potential terrorists.
8. After listening to "both sides" of the debate, the public will give up hope that anything can be done and accept that routine gun massacres are just a part of American life.
9. Donald Trump tweets something crazy and Americans forget about hundreds of people strewn across the Vegas strip blown to pieces by a crazed gunman.
10. The next gun massacre happens and we repeat the cycle.