because In the Quran, we read that Allah is Compassionate and Merciful. He is Kind, Loving, and Wise. He is the Creator, the Sustainer, the Healer. He is the One who Guides, the One who Protects, the One who Forgives. There are traditionally ninety-nine names, or attributes, that Muslims use to describe Allah's nature.
Something may be said to be evil when it takes human life, or if it causes humans to suffer in an inhumane way. Enslaving people against their will would be inhumane and evil. The massive killings of wars are an evil. One person killing another innocent person is evil.