The issues are not mutually exclusive: our laws and regulations have regularly allowed people, whether they are jihadists or otherwise mentally troubled, nearly unfettered access to powerful firearms. Could the San Bernardino shooters still have gotten ahold of their weaponry under stricter gun laws/regulation? Sure; no laws are foolproof, and there will always be people who find a way to achieve their goals. But why make it so easy to obtain assault rifles and thousands of rounds of ammunition?
To put it into perspective, here is a (non-comprehensive) list of high-profile shootings where the perpetrators bought there weapons legally and easily:
Roanoke
Charleston
Fort Hood 1
Fort Hood 2
Washington Navy Yard
Sikh Temple (Wisconsin)
Aurora
Tuscon
Virginia Tech
Isla Vista
San Bernardino
What are the odds that every single one occurs if the perpetrator has difficulty obtaining a gun? If even one person in these events lives because the shooter could only buy one pistol, or didn't have access to a high capacity magazine, is it worth it? Of course it is
Additionally, the guns used in Sandy Hook and Columbine, though technically illegal (because they were stolen/illegally held), were otherwise purchased legally by their original owners; no one needs to amass that amount of firepower, and doing so is at best pointless and at worst homicidal (lest the wrong person get access to them).
This Onion article is hopelessly true:
http://www.theonion.com/article/no-way-to-prevent-this-says-only-nation-where-this-36131