"There's nothing new under the sun, It's all been seen before."

Sherlock means that murder isn't a new thing and has been around since man itself began. In all this time, there are so many ways these people could have been killed and that by knowing the history of the death or knowing another similar murder, finding out the killer could be relatively easy. I think that it is true, especially nowadays because its so frequent and the people who murder tend to see monkey do, like the Newton shooting and the man at McNair who barely got talked down from doing the same.