In the garden what if God had said do not kill rather than do not eat from the tree...how would the serpent?

Yes the devil Satan does that all through history to those who are out of their minds.

Jesus said he is liar and the Father of lies. Jesus went on to say that the devil comes to steal, kill and destroy us.

So we are to be aware of this each day and night not to be drawn into his lies.

Well the tree was of 'the knowledge of good and bad'.... Wouldnt that be included under that? God telling them not to eat from the tree was, in essence, him letting them know that he and only he could rightfully exercise his sovereignty and therefore tell them what was right and wrong. Satan questioned Eve on that, he gave her a big head, he lead her to believe she could act independently, she could decide for herself what was right or god, she didnt need god, or like he said, she could become like God.

He could have given them contraception its the same as murder.

Death only entered the world after sin, so it wouldn't happen.

There was no death yet.