However insignificant the part of the free will seems in man's actions and however slight a sin unbelief may seem to some at first sight it is a denial and negation and therefore destructive. and deserving eternal punishment. Remember that we likened the part of the free will in man's actions to turning on or off the switch to illuminate or darken a room. So by turning off a switch you can reduce a whole city to darkness. To cite other examples by lighting a match you may burn to ashes in a few minutes a huge magnificent palace which hundreds of workers constructed over several years. Remember that it was a single bullet fired by a Serbian which ignited the fire of the First World War and led to millions of lives lost and massive destruction.
Also suppose there is a garden with flowers and trees of all kinds on the branches of which birds sing and in which all kinds of animals live. All those plants and animals subsist on the water reaching them through canals and a person is in charge of irrigating the garden by letting the water flow to the canals from a big reservoir. What kind of sentence do you think would be appropriate if this person were to let the garden with all the plants and animals therein die of thirst by simply turning off the flow of water? The act of unbelief is equivalent to such an act but on the scale of the creation as a whole.