Obviously the kid wanted a large body count he was getting guns regardless
but if he stole them from his mom, he pretty much got them the easiest way possible. it seems a stretch to assume he would have got them some other (much more difficult) way. maybe he would have tried, but got discouraged, or even busted
not to mention, just him knowing the guns were there, easily accessible in the house, could have helped him come up with this 'plan'. in other words, if he didn't have easy access to guns, he may not have ever even really thought to do all this (that's all pure conjecture of course)
edit: I know that was the case when I was a youngster at my friend's house where I knew his pops had guns. we knew they were there, we wanted to shoot them, and we did
