Read the passage. There are persons too who see not the full extent of the evil that threatens them; they solace themselves with hopes that the enemy, if they succeed, will be merciful. It is the madness of folly to expect mercy from those who have refused to do justice; and even mercy, where conquest is the object, is only a trick of war: The cunning of the fox is as murderous as the violence of the wolf; and we ought to guard equally against both. What is Thomas Paine’s main point in this passage from The Crisis? Colonists need to rely on their cleverness rather than their strength

It is made very clear by him that mercy from the invaders is merely a trick to tame a recently defeated and conquered . There's nothing merciful about letting an innocent live since the ones who invaded, raided and conquered are the criminals in the first place.

In the end, the mercy is only an illusion of "an end to the war" and sudden peace, but what really is sickens Thomas Paine, as he says, "It is madness of folly to expect mercy from those who have refused to do justice".

In the end, since the mercy is just a trick, it means that at any point, the slaughter can continue since "the cunning of the fox is as murderous as the violence of the wolf".