One only (εις). No "only" in the Greek, but εις here excludes all others but God.

The lawgiver (ο νομοθετης). Old compound (from νομοσ, τιθημ), only here in N.T. In Psalm 9:20. Cf. νομοθετεω in Hebrews 7:11; Hebrews 8:6.

To save (σωσα, first aorist active infinitive of σωζω)

and to destroy (κα απολεσα, first aorist active infinitive of απολλυμ to destroy). Cf. the picture of God's power in Matthew 10:28, a common idea in the O.T. (Deuteronomy 32:39; 1. Samuel 2:16; 2. Kings 5:7).

But who art thou? (συ δε τις ει;). Proleptic and emphatic position of συ (thou) in this rhetorical question as in Romans 9:20; Romans 14:4.

Thy neighbour (τον πλησιον). "The neighbour" as in James 2:8.