Looks like the shock of losing everything sent Neville off the deep end. Kind of sad. But Neville should have finished Draco off first. A penknife would puncture a lung, but would it really kill him off that fast? Since you said penknife, I picture something with maybe a 2 in. blade. But killing Neville so easily, it must have been a "pocketknife" with a 3 or 4 in blade or longer. Good action! Those aurors weren't too smart, not covering their backs or watching the map for others.