@Salfaro:
The assassin assumes that the MC only stays put in the dungeon because he believes there are extremely strong fighting forces amongst the humans.
But if all human adventurers would fail at the start when facing the dragon zombie she thinks the MC would realize that humanity's fighting power is far lower than what he assumes and might even realize that he himself is already far more powerful than any other human adventurer.
So she fears that the MC will stop defending and go on the offensive by using his full power in order to destroy humanity.
That is the part we see her she invisioning about the MC sending an attack magic to the human country that would cause that kind of huge explosion.
Therefore she tries to prevent the MC from finding out about the weak human fighting power by making the excuse about collecting energy from even "weak" human adventurers that can only clear some "easy difficulty" floors.
The "weak" human adventurers she talks about to the MC would in reality be the top ranking adventurers and fighting power humanity has.
She wants the MC to think those are just some "weak" adventurers in order for him to stay hidden inside the dungeon on defense thinking that some "strong" human adventurers might just be waiting for an opening in the dungeon defenses.
That is her plan as to why she "helps" the MC.