I don't deny that there was a lot of left leaning extremism and terrorism in the 60s, 70s and sometimes into the 80s. However that is a lot less prevalent now, and the most likely domestic terrorism groups in the US now are right-wing white nationalists and Islamist extremists.
Bill Ayers association with President Obama is however blown out of proportion
The "he was insane" copout is just that, a copout. There have been many, right-wing, white supremacist, nationalist, anti-government attacks, here are the main ones since 2010:
2010 Andrew Joseph Stack III
2010 James J. Lee
2012 Wade Michael Page
2013 Everett Dutschke
2013 Paul Anthony Ciancia
2014 Jerad and Amanda Miller
2015 Charleston church shooting: Dylann Roof
2015 Colorado Springs shootings: Robert L. Dear