This is not an easy topic to write about as a lot of responsibility and care goes into being the ruler of the world. But it is a very thought provoking prompt.
I'm going to base my thoughts that revolve around responsibility and care, my spiritual beliefs and my values in life.
If I ruled the world ...
I would provide the ability for all who exist to live a harmonious and decent life.
I don't think I would have the right to take away an individual's right to live a certain way but I think it would be my duty to ensure that I provided the ability for all to live well, should they want it.
I'm almost anticipating that most who read this would be saying, why wouldn't they. But who knows what each individual soul is here to experience. Spiritual studies infer that lives lived in hardship usually provide greater progress at the soul level.
I would promote the brother(sister)hood of mankind
I truly believe that all of creation is inter-connected. All the souls who have arrived on earth are here for a greater purpose than just our individual lives, God's great plan, a plan that we cannot comprehend in the context of the cosmos.
I truly believe that peace is the way and that can be achieved only when we learn to live together, accepting each others differences and thoughts, when we can understand each others pain and show compassion for the less fortunate, and when we truly learn to appreciate the splendour of all God's creations.
So I would try to bring about an understanding of that.
I would provide justice for all - the law of Truth shall prevail
While I truly believe peace is the way and that the act of war cannot bring about peace, my spiritual quest has led me eventually back to my roots in India to the great epic legends of India.
The Ramayana and the Mahabharata upon which the foundations of the Hindu religion lie, depict great wars (to the scale of the predicted Armageddon) that have been commanded and led by Lord Vishnu (who the Hindu's believe is the preserver and sustainer of life) in His human incarnations (Lord Ram and Lord Krishna), to bring about peace and goodwill to humanity and to remove evil.
Everything in the world and even the cosmos is balanced - the ebb and flow of tides, day and night, water and fire, earth and air, male and female, good and evil, harmony and destruction, powerful and powerless, life and death, all nicely summed up in the yin and yang.
And as all things balance in life, when the cosmos becomes imbalanced, and where sense and sensibility cannot rectify that imbalance then to provide justice for the greater good, eradication of evil enough to bring back the balance of nature becomes a necessity.
The Hindu scriptures predict that Lord Vishnu will come again in human form to restore peace and harmony to the world again. This avatar of the Lord will however be merciless in eradicating evil and extremely benevolent to those who hold the essence of truth and goodness. He will end the age of destruction and begin the age of Truth.
I think it would my duty as the ruler of the world to uphold the truth and to punish evil thus bringing about justice for all.
Whether we look at eradication via wars or via penalties (like sanctions) in the end the act is the same. Innocent lives suffer in either circumstance. Our inter-connectedness also therefore means that those who eradicate also suffer. I have no answer on how to prevent that except to pray to the Lord to provide his shelter and grace to the innocent and to forgive the necessary act which leads to the destruction of life that He has created.
Peace and Blessings to all.