Terrorists don't tend to leave much doubt whether their actions were intended as terrorism. That's why I don't think there's much chance this is an al-Qaida plot.
These are thrill kills, I think.
I think September 11 has taught AQ a serious lesson. They had 3 attacks plan (apparently) and were unable to carry out the second 2 as a result of the 'round-up' done of suspects afterwards. I would not put it past the AQ to send out a couple of nuts on a rampage like this and only reveal their action aftewards. Otherwise it would prompt another round up making it harder for them to operate.
Or it could be a couple of thrill seekers whacked out on some schizoid mission.