Preface: I am no expert, this is just my understanding.
Weapons that are illegal/considered war crimes fall roughly into categories of:

A. Indiscriminate - kill soldiers and non-combatants/civilians alike (eg. Land mines, incendiary, cluster bombs, etc)

B. Cruel - especially painful ways to die or designed to cause ongoing suffering and maiming. (Eg: gas/chemical warfare, dirty bombs, etc)
A lot of weapons tick both of those boxes, and there are possibly more i am unaware of.