Why did God say this to Isaiah?

The context makes it clear the reference to evil is the evil that comes against the wicked as a judgment.

It does not refer to evil in the absolute sense.

other versions render the word "calamity".

Remember that this prophecy of Isaiah was recorded some 200 years before the event. Upon hearing it some may have wondered, ‘Does Jehovah truly have the power to fulfill it?’ As history testifies, the answer is yes. Jehovah explains why it is reasonable to believe that he can accomplish what he says: “Forming light and creating darkness, making peace and creating calamity, I, Jehovah, am doing all these things.” (Isaiah 45:7) Everything in creation—from light to darkness—and everything in history—from peace to calamity—is subject to Jehovah’s control. Just as he creates the light of day and the darkness of night, so he will make peace for Israel and calamity for Babylon. Jehovah has the power to create the universe, and he also has the power to fulfill his prophecies. That is reassuring to Christians today, who closely study his prophetic word.

The Meaning of Jehovah’s Bringing Evil. Rightly, Jehovah brought evil or calamity upon Adam for his disobedience. Hence, in the Scriptures, Jehovah is referred to as the Creator of evil or calamity. (Isa 45:7; compare KJ.) His enforcing of the penalty for sin, namely, death, has proved to be an evil, or a calamity, for mankind. So, then, evil is not always synonymous with wrongdoing. Examples of evils or calamities created by Jehovah are the Flood of Noah’s day and the Ten Plagues visited upon Egypt. But these evils were not wrongs. Rather, the rightful administration of justice against wrongdoers was involved in both cases. However, at times Jehovah, in his mercy, has refrained from bringing the intended calamity or evil in execution of his righteous judgment because of the repentance on the part of those concerned. (Jon 3:10) Additionally, in having a warning given, Jehovah has undeservedly provided opportunities for the practicers of bad to change their course and thus to keep living.—Eze 33:11.

Remember that this prophecy of Isaiah was recorded some 200 years before the event. Upon hearing it some may have wondered, ‘Does God truly have the power to fulfill it?’ As history testifies, the answer is yes. God explains why it is reasonable to believe that he can accomplish what he says: “Forming light and creating darkness, making peace and creating calamity, I, am doing all these things.” (Isaiah 45:7) Everything in creation—from light to darkness—and everything in history—from peace to calamity—is subject to God's control. Just as he creates the light of day and the darkness of night, so he will make peace for Israel and calamity for Babylon. God has the power to create the universe, and he also has the power to fulfill his prophecies. That is reassuring to Christians today, who closely study his prophetic word.

The Meaning of Jehovah’s Bringing Evil. Rightly, Jehovah brought evil or calamity upon Adam for his disobedience. Hence, in the Scriptures, Jehovah is referred to as the Creator of evil or calamity. (Isa 45:7; compare KJ.) His enforcing of the penalty for sin, namely, death, has proved to be an evil, or a calamity, for mankind. So, then, evil is not always synonymous with wrongdoing. Examples of evils or calamities created by Jehovah are the Flood of Noah’s day and the Ten Plagues visited upon Egypt. But these evils were not wrongs. Rather, the rightful administration of justice against wrongdoers was involved in both cases. However, at times Jehovah, in his mercy, has refrained from bringing the intended calamity or evil in execution of his righteous judgment because of the repentance on the part of those concerned. (Jon 3:10) Additionally, in having a warning given, Jehovah has undeservedly provided opportunities for the practicers of bad to change their course and thus to keep living

In the context of the verse Jehovah God was in a prophetic way announcing his judgement againt the kingdom of Babylon who had taken Israel into captivity due to their disobedience. Cyrus was the leader of the Medo-Persian army that fulfilled this prophecy in the book of Daniel chapter 5. In the book of Daniel 6:28 Darius the Mede and Cyrus are co-rulers and it was Cyrus that issued the decree for the Israelites to return and rebuild their kingdom at Ezra 1:1-4.

The statement was an affirmation that the earth and all things in it are not insurmoutable to Jehovah's power to fulfill His promises.

He was seriously stoned