As it stands, the Geneva convention only applies to acts when both countries are signatories, or one is, and the other follows the convention (which Hamas unfortunately does not): https://en.wikipedia.org/wiki/Geneva_Conventions#Common_Arti...

Is there another treaty which proscribes perfidy, which Israel has ratified?

Why was it decided that feigning of civilian, non-combatant status is bad? because it led to death of civilians who had no part in the fight; pretending to be your enemy's civilians bring no such issue. Although assassinating a patient is also not kosher it less relevant to the discussion about use of uniforms.