Why did the same Arjuna who didn’t want to kill his own relatives before the war want to kill Yudhishthira?

From: Apurva Pradhan

 “Gandiva Vow”. Arjuna was ready to kill his own brother to keep a Vow. The same Arjuna was earlier ready to live like a mendicant instead of killing his kinsmen. Kindly help me understand these two seemingly opposing sides of Arjuna’s nature.