48 Then the Jews answered and said to Him, “Do we not say rightly that You are a Samaritan and have a demon?”
49 Jesus answered, “I do not have a demon; but I honor My Father, and you dishonor Me.
50 And I do not seek My own glory; there is One who seeks and judges.
51 Most assuredly, I say to you, if anyone keeps My word he shall never see death.”
Jesus said their Father was the devil. They responded with blasphemy (v. 48). Unable to answer Him, they resort to name-calling.
To call a Jew a Samaritan (v. 48) was slander. It was calling him an outcast, a wicked apostate. They said this in response to Jesus' statement that Abraham wasn't their father.
To call Jesus a demon was a sin that couldn't be forgiven (Matthew 12:24; Mark 3:22). They said this in response to Jesus' statement that the devil was their father — they said He was a demon because He was lying (they claimed).
If Jesus sought His own glory (v. 50), their accusations would have hit the mark, but He sought only the Father's glory so their words were an insult to God. And the Father will judge those who dishonor the Son — and His judgment is death (not physical death, but eternal, spiritual death).
one who seeks — the Father seeks glory for the Son.
Jesus wasn't concerned with their opinion of Him. The One who judges would take care of them.