Why did Elijah kill all the prophets of baal?

Those prophets were 'sitting on the fence' (according to The Message paraphrased Bible).

Elijah gave them a chance. Elijah told them to call upon their gods to 'light the fire'. They prayed and cried and called. But their god was 'not a god'. Elijah mocked and challenged them. Told them to call louder. Maybe their god was asleep; calling louder might wake their god; or on a trip, or perhaps sitting on the toilet (Contemporary English Version).

Then Elijah had them bring buckets of water on the altar. Elijah prayed once, and boom, fire from heaven consumed the altar and everything. If that wasn't evidence that God approved of Elijah as his true prophet.

Elijah told 'the people' to grab the prophets. So 'the people' did as told. They could have refused.

Then Elijah had them killed. 1 Kings 18:40 according to the Amplified Bible adds: "[as God's law required]". So really that's the reason WHY Elijah had them killed: God's law required it. Elijah and 'the people' obeyed. I guess God didn't want any 'false' prophets around.

The Bible states that in the future God will 'cut off' all evildoers. Why? Because they are 'evildoers'. It's really that simple. God requires obedience. At least that's what's in the Bible.