God doesn't make mistakes, and is sinless. If God doesn't make mistakes and He is sinless then why did He make people sin and cause evil to exist in the world?
God did not create sinners, killers or thieves. God created man in His own image "So God created man in His own image; in the image of God He created him; male and female He created them" (Gen 1:27). Being created in the image of God meant that God gave us the free will. Man unfortunately chose, by his own free will, to disobey God and break His commandment. The day man ate of the forbidden tree, sin entered the world. All the evil we see today is the result of the sin of man. A man without a free will is not a perfect man, God intent for us is to be perfect like Him "Therefore you shall be perfect, just as your Father in heaven is perfect" (Mt 5:48).