Why didn’t God kill off Eve before she could influence Adam?

Even though God is Omniscient, knowing all future events, out of love He still created humans to freely choose. Our purpose in life is to have a relationship with God. All through the creation account we see God saying “it is good” over and over again until He mentioned Adam being alone. Then God said, “it is not good” for the very first time.

Why have we all sinned in Adam if Eve was the first one to sin? 

1 Timothy 2:13-14

For it was Adam who was first created, and then Eve. And it was not Adam who was deceived, but the woman being deceived, fell into transgression.

Genesis 3:9-10

Then the Lord God called to the man, and said to him, “Where are you?” He said, “I heard the sound of You in the garden, and I was afraid because I was naked; so I hid myself.”

The fact that God called out to Adam and not Eve is telling. Adam is the first person in history and in his federal headship represented  the whole human race. 

1 Corinthians 15:21-22

For as by a man came death, by a man has come also the resurrection of the dead. For as in Adam all die, so also in Christ shall all be made alive.