incorrect on both counts.
1) there is a thing called reloading. the VT shooter killed more people with two pistols than lazna did with his AR-15 rifle.
2) nancy lanza did not own the most lethal weapon allowed by law. first, fully automatic weapons are legal to own, but they are extraordinarily expensive to acquire.
second - you need to define "lethal". is it rate of fire? if so, any semi-auto is equally effective, rifle or pistol. is it magazine capacity? in that case, there are plenty of 50+ round aftermarket magazines available that clearly were not used. is it the bullet? the 5.56/.223 are not very deadly rounds compared to other rifle cartridges which fire larger bullets at higher velocities and have deeper penetration. even some pistol rounds have better penetration. is it a combination of factors? which ones are more important than the others?
so "most lethal", until you define lethality in a technical and comparative manner, is a completely subjective term. what you might think is "most lethal" - in this case, an AR-15 rifle, may not be the "most lethal" thing that a person well versed in firearms (or even relatively poorly versed, such as myself) knows.