Arjun was a proclaimed warrior, but due to his righteous nature, he tried to avoid violence whenever possible. This side of his character is well revealed in an incident that took place towards the end of the Mahabharat war.
Duryodhana was badly injured, and only a handful of warriors were remaining on the Kaurava side. Ashwatthama was one of them. He wanted to avenge his father Dronacharaya and his friends Kauravas' death. He sneaked into the Pandava camp at night while everyone was fast asleep and went on a carnage. Several warriors including Draupadi’s five sons were killed in this attack. The Pandavas, along with Draupadi and Shree Krishna were away that night. When they returned to the camp, they were devasted. Arjun went after Ashwatthama, caught him and threw him at the feet of Draupadi, who was grieving the death of her young sons. Bheem was raged and wanted to kill Ashwatthama right way, but Draupadi was a soft-hearted lady of forgiving nature. She said that Ashwatthama was their Guru Drona’s son and a Brahmin, therefore, they should forgive him. Arjun was unsure what to do next and wanted guidance from Shree Krishna, who then said, “A respect-worthy Brahmin must be forgiven even if he may have temporarily fallen from virtue.  But a person who approaches to kill with a lethal weapon must certainly be punished.”  Arjun understood what the Lord meant; he did not kill Ashwatthama. Instead, he cut his brahmin’s tuft, removed the jewel of immense power from his forehead, and let him go.
ṛitvikpurohitāchāryair mātulātithisanśhritaiḥ
bālavṛiddhāturair vaidyair jñātisam bandhibāndhavaiḥ
(Manu Smṛiti 4.179)
“One should not squabble with, the Brahmin who performs the fire sacrifice, the family priest, teacher, maternal uncle, guest, dependent children, elders, doctor or relatives.”
Arjun was aware that killing one’s own relatives was a great sin. He said to Shree Krishna, “Greed has blinded them (Kauravas) and they do not realize that it is a great sin to kill their own relatives and friends, but why should we do the same thing, when we can avoid this transgression?”