Roof knocking is using non explosive ordinance. It is, by definition, not a bomb. Attacking its use is wild, its a tactic that saves civilian lives, even if you disagree with the validity of the target.
> that means you think that Hamas sent these people on the roof to let the Israeli army execute them, not even to use them as a human shield.
Yes. That is what human shielding is. The unfortunate reality is that it is irresponsible to completely stop attacks when human shielding is used, as it encourages further use of the practice. Just like blaming Israel for all of those death is also encouraging Hamas to further use the tactic.
> What would be the military objective here?
Hamas has been very clear that deaths of their civilians further the Palestinian cause by causing the world to turn on Israel. The objective here is clear.
Now I turn it back to you: what is the military purpose of roof knocking?
No double tapping doesn't make sense here. You wouldn't use a non-explosive ordinance if the goal was "double tapping"