Krishna is impartial, but He specially protects the surrendered!

Verse 1.7.25: Krishna comes again and again to annihilate the miscreants and protect the devotees. Is God partial or not? Srila Narada muni explains that He is not partial. He gives Himself equally to the devotees and non-devotees. But whoever is surrendered to Him, He protects them, even if it is Hiranyakashyapu.

Here, Arjuna and Ashwatama both are spirit souls, jivas. But Arjuna is surrendered and Ashwattama is not. Therefore Krishna protects Arjuna.