Hamas definitely do not abide by international law, however the world rightly expects that a sanctioned state military would abide by international law. IDF have repeatedly broken dozens of international laws so far.
Firing on unarmed humans carrying a white flag is just one. Even if the hostages they mistakenly killed had been Hamas, by international law you cannot fire on enemy combatants if they are unarmed and with a white flag.
Palestinians cannot “flee to safety” because Israel has besieged Gaza. There is no way in or out unless your name is on an Israel approved list at Rafah crossing. Very few wounded civilians have been allowed by Israel to leave, less than 1%.
The Hamas tunnel system, like any undergound tunnel system go under civilian buildings. Every metro in every city goes under civilian infrastructure. The mere presence of tunnels under civilian buildings is not considered using human shields. There would have to be access to the tunnels from inside the civilian buildings themselves. Finally, just because your enemy breaks international law, that doesn’t mean you can too. It doesn’t become a free for all, you still have to avoid civilian casualties as much as possible.
The fact IDF gunned down unarmed hostages, shirtless, and waving a white flag, is evidence that Palestinian civilians, UN workers, Red Cross and so on are likely not lying about the indiscriminate nature of IDF’s ground offensive.