Pay close attention to John 8:44 "he was a murderer from the beginning" this is a reference to Genesis when the serpent (Satan) tempted Adam and Eve with the fruit of the tree of knowledge of good and evil.
For context.
Genesis 2: 16-17
16 And the Lord God commanded the man, saying, Of every tree of the garden thou mayest freely eat:
17 But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.
Here is the first lie Satan told.
Genesis 3:1-4
3 Now the serpent was more subtil than any beast of the field which the Lord God had made. And he said unto the woman, Yea, hath God said, Ye shall not eat of every tree of the garden?
2 And the woman said unto the serpent, We may eat of the fruit of the trees of the garden:
3 But of the fruit of the tree which is in the midst of the garden, God hath said, Ye shall not eat of it, neither shall ye touch it, lest ye die.
4 And the serpent said unto the woman, Ye shall not surely die:
The Murder is that of Adam and Eve. Which caused Adam and Eve to lose their immortality. Effectively murdering them by causing them to be able to die.