It's not the occupation of the West Bank and Gaza which is illegal under international law, it's the settlements, so the policeman arguably had the right to be there as part of the legal occupying force. The fact that the Palestinians don't like it is another question altogether and is what bronxite was referring to as the big difference, I believe. And, even if the policeman was part of an illegal settlement, is the internationally recognised legal punishment for "illegally settling" to be shot to death by vigilantes? Last time I looked, international law required a court and a trial.
Netanyahu condemns West Bank arson attack, calling it 'terror' (Haaretz)
from the article: PA to fall by end of year, says Palestinian minister