Personally I am on the side of civilians whether they are Israeli or Palestinian or Druze or Bedouin. I am also on the side of the doctors, nurses, and humanitarian aid workers. These are who I consider the Good Guys. "Look for the helpers." - Mr Rodgers.
But to cover your question, there's two separate issues here:
the claims of the belligerents
feelings about what is happening now in the Israel Gaza war
The past claims of the belligerents
I'm not going to go into this, but but it goes back to history and it's where the side-taking on the sides of the belligerents mostly comes from, because different people have different interpretations. Legally speaking Palestine is occupied by Israel (West Bank) and blockaded by Israel (Gaza). No one can get in or out, it's effectively controlled by Israel.
What is happening now
What is happening now is a "war" between the IDF and Hamas. People like me, who are rooting for civilians are upset about:
proportionality (so far about 33,000 Palestinian deaths vs 1,400 Israeli deaths)
mortality (currently over 1.5% of the population of Gaza have been killed)
civilian mortality - high numbers of children being killed (according to aid agencies, one is killed or injured every 10 minutes)
high numbers of humanitarian workers and medical staff being killed
inappropriate weapon choices (hundreds of 2,000lb bombs are being dropped, for reference the US used just one in its war against Isis.
widespread infrastructure destruction, particularly of hospitals and other important buildings
mounting evidence of starvation in the civilian population due to blockade of aid
Personally I am against anyone treating a civilian population like this for any reason, and I believe it amounts to war crimes. I was against the killing of civilians in Myanmar, Rwanda, East Timor, and I am against it now.
Side note: The median age in Gaza is 18, meaning literally half the people in it are still children.
Edit: I haven't given any sources cos lazy. I am happy to give them to OP or anyone in good faith (there are probably a bunch on the post history of my main which is livus@kbin.social). However, I do not speak sealionese.