Because he obtained a gun. Illegally.
"the 2nd amendment is about the militia keeping their arms in their homes
its about no standing army but every swinging dick owning one in case there's a call to arms
its about the little guy, your state, having the means to shoot down the big guy, your feds, when the big guy gets too big
its about states rights and the rights of the individual
its old and outdated because we are big and corporate and these days we want a big damn army to kill the chinee and the rus
its time for repeal"
I agree with everything bagchucker just posted in the 1st 4 lines. (quoted above)
However, there just isn't enough citizens willing to go along with the notion in line 5, AND even fewer willing to talk repeal of the 2nd Amendment.
Exactly because that 2nd Amendment is about (partly) (as bagchucker points out) the "rights of the individual".
Thus we're stuck with the Amendment as written. Seems to me the ONLY answer is a redefinition of what a "well regulated militia" is allowed to maintain at home!