Partha! You honour always the meritorious people. In case, you kill Vrishasena, son of Karna before his eyes in order to keep your commitment, he will definitely remember the advice given him by Dronacharya, Bhishma and Vidura time to time under influence of agony caused by separation of his son (i.e. so killed by you).
त्वं चेत् कर्णसुतं पार्थ सूतपुत्रस्य पश्यतः। प्रतिज्ञावारणार्थाय निहनिष्यसि सायकैः॥ हतं कर्णस्तु तं दृष्ट्वा प्रियं पुत्रं दुरात्मवान्। स्मरतां द्रोणभीष्माभ्यां वचः क्षत्तुश्च मानद॥