1. Let's agree, both sides haven't been very nice to each other in the past.
2. On this occasion the action of the military wing (apparently not sanctioned by the political wing who reportedly mostly live outside of the area) was one designed to strike terror. It has provoked worldwide feelings of revulsion and disgust.
3. Whatever has happened in the past cannot and does not justify what happened. No "What about" or "But they did this first".
and so to go back to the original question, people can be Pro-Israel in this situation because it was a terrorist act, even if more generally they might have been sympathetic to the Palestinian cause.