A rash of recent assassinations have brought on congressional scrutiny and concern among law enforcement agencies who are wary of an age of political polarization turning deadly. But experts say the violence is as much a byproduct of the times as it is the easy accessibility to DIY murder tradecraft, evident in some high-profile slayings of late. So while the willingness to commit these acts has certainly increased, the tradecraft to pull them off has never been more obtainable. “Political polarization, combined with the idea that one’s opponents are irredeemably evil and that there are no other legal avenues to...