Dear Sami I do also believe on this notion that this world is not only based on mercy and kindness. Ruthless nature, cruelty and animalistic barbarity are also included as a counter current of mercy. Now just because of the presence of this destructive force of life would it be wise to call the whole system of world unjust? I think we have to revisit our position on this focal point with other perspectives.
Can you recall the distinctive concepts of Apollonian and Dionysian in ancient Greek mythology? The former one is loosely based on the creative power of universe and the later one is based on the destructive force and hence the universe cant exist with the absence of either one of the above mentioned forces.
Remember in Islamic theology and corpus it is never mentioned explicitly that God is governing the world with the force of kindness alone. Sooner this point could be understood the better for anxious souls like you and me who want to reconcile the Mercy of God with the prevalent destructive forces of the world. Why dont we become the supporters of the power of constructive force instead of ruing the dominance of destructive forces instigated by the followers of Satan?
If we could raise the level of faith in our souls then why our hands couldnt soften the hard element of Iron like Prophet David used to do in his life? Or could melt the forces of evil with stones like tiny creatures of Martins (Ababeel birds) did with the overwhelming elephant force of Abraha centuries ago? Or can transform the mere dust of earth to blind the raging forces of polytheists of Badr?
One more reality which Islamic eschatology has alluded to in times to come (In End times) that when this world will be filled with justice and peace then even the brutal instincts of beasts will be tamed and transformed into the nature of kindness e.g. Snakes will not bite other creatures, lions and sheep will drink side by side from the brook.
We have to develop our trust and faith on God Almighty so that with merely our vision we could be able to dissolve the strongest imaginable evil from the face of the world, a trait which God will bestow to Prophet Essa AS in end times as well. Just whining and getting overly skeptic about the prevalent evil nature of the world could not lead us to any constructive conclusions my friend.