It is my understanding that a major reason Palestians voted for Hamas was their promise to bring security to the cities, etc.. The situation in Gaza was close to total anarchy. Abbas hasn't done anything, so someone had to, regardless what their motivating factor might be.
Israeli soldier collapses, dies from heatstroke during visit to Jerusalem's Old City (Haaretz)
from the article: Gunfight erupts between Hamas and Fatah forces in Gaza