Here's an interesting thought. Although this does not represent the mass shootings/stabbings, it does represent the safety in the US over time.
1975 US Population - 213,124,000
1975 Total Crimes (Murder, rape, assault, robbery ect...) - 11,292,400
2000 US Population - 281,421,906
2000 Total Crimes - 11,608,072
2011 US Population - 311,591,917
2011 Total Crimes - 10,266,737
So what do you notice? Over the years as population increases, the number of crimes decrease. So do we live in a safer country now than 30 years ago? Statistics would say yes.
Here's my source.
http://www.disastercenter.com/crime/uscrime.htm