First take into account the context of those two verses that are out of context enough to cause them to appear contradictory. If you read the following passage carefully it will be revealed how Jesus will judge
John 5:18-30 (KJV)
18 Therefore the Jews sought the more to kill him, because he not only had broken the sabbath, but said also that God was his Father, making himself equal with God.
19 Then answered Jesus and said unto them, Verily, verily, I say unto you, The Son can do nothing of himself, but what he seeth the Father do: for what things soever he doeth, these also doeth the Son likewise.
20 For the Father loveth the Son, and sheweth him all things that himself doeth: and he will shew him greater works than these, that ye may marvel.
21 For as the Father raiseth up the dead, and quickeneth them; even so the Son quickeneth whom he will.
22 For the Father judgeth no man, but hath committed all judgment unto the Son:
23 That all men should honour the Son, even as they honour the Father. He that honoureth not the Son honoureth not the Father which hath sent him.
24 Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.
25 Verily, verily, I say unto you, The hour is coming, and now is, when the dead shall hear the voice of the Son of God: and they that hear shall live.
26 For as the Father hath life in himself; so hath he given to the Son to have life in himself;
27 And hath given him authority to execute judgment also, because he is the Son of man.
28 Marvel not at this: for the hour is coming, in the which all that are in the graves shall hear his voice,
29 And shall come forth; they that have done good, unto the resurrection of life; and they that have done evil, unto the resurrection of damnation.
30 I can of mine own self do nothing: as I hear, I judge: and my judgment is just; because I seek not mine own will, but the will of the Father which hath sent me.
Jesus will be the One who judges (decides) who will be resurrected to safety,to eternal life with him. A thousand years later he will bring up the evil dead. The delay is to lump all past evil doers in with the who rebel after the end of Jesus' millennial reign when Satan is loosed to deceive many. Until then Jesus said he judges on the basis of hearing then saying/doing what the Father tells him to do. He doesn't seek his own will, but does the will of the Father.
Now let's look at the context around Jn 8:15.
John 8:3-16 (KJV)
3 And the scribes and Pharisees brought unto him a woman taken in adultery; and when they had set her in the midst,
4 They say unto him, Master, this woman was taken in adultery, in the very act.
5 Now Moses in the law commanded us, that such should be stoned: but what sayest thou?
6 This they said, tempting him, that they might have to accuse him. But Jesus stooped down, and with his finger wrote on the ground, as though he heard them not.
7 So when they continued asking him, he lifted up himself, and said unto them, He that is without sin among you, let him first cast a stone at her.
8 And again he stooped down, and wrote on the ground.
9 And they which heard it, being convicted by their own conscience, went out one by one, beginning at the eldest, even unto the last: and Jesus was left alone, and the woman standing in the midst.
10 When Jesus had lifted up himself, and saw none but the woman, he said unto her, Woman, where are those thine accusers? hath no man condemned thee?
11 She said, No man, Lord. And Jesus said unto her, Neither do I condemn thee: go, and sin no more.
12 Then spake Jesus again unto them, saying, I am the light of the world: he that followeth me shall not walk in darkness, but shall have the light of life.
13 The Pharisees therefore said unto him, Thou bearest record of thyself; thy record is not true.
14 Jesus answered and said unto them, Though I bear record of myself, yet my record is true: for I know whence I came, and whither I go; but ye cannot tell whence I come, and whither I go.
15 Ye judge after the flesh; I judge no man.
16 And yet if I judge, my judgment is true: for I am not alone, but I and the Father that sent me.
Again, Jesus pointed to what the Father said, which he obeyed. He came as the light of the world, not Judge. He will return as Judge.
I hope that helps you see there is no contradiction.
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