If God never fail why did he go from killing people mercilessly in the old testament to sending jesus?

You're right a perfect being wouldn't have to change his plans. Therefore, God didn't change his plans. His purpose and plans are still in affect. It's that you haven't gain accurate knowledge of the Scriptures to truly understand His purpose for mankind and why certain events taken place during that time.

Jesus was always his plan...he changed nothing

and He will STILL judge the living and the dead

God executed divine justice and mercy. God does not change.

Cherry-picking the OT makes you look like an idiot.

"It is not the will of God that the evil should parish, but that he should turn from his evil ways and live." (Ezekiel)

"Come, let us reason together. Though your sins are like scarlet they shall be white as wool." (Isaiah)

He didn't change plans that was all part of his plan

Where did he ever claim to be perfect?

lol where did you copy&paste this question from? Because you obviously never read the Bible.

to show you whos boss

thats right, god can kill