Charles Spurgeon Commentary 1 Samuel 20:7-8

"If he say thus, It is well; thy servant shall have peace: but if he be wroth, then know that evil is determined by him. Therefore deal kindly with thy servant; for thou hast brought thy servant into a covenant of Jehovah with thee: but if there be in me iniquity, slay me thyself; for why shouldest thou bring me to thy father?" — 1 Samuel 20:7-8 (ASV)

If he say thus, It is well; thy servant shall have peace: but if he be very wroth, then be sure that evil is determined by him. Therefore thou shalt deal kindly with thy servant; for thou hast brought thy servant into a covenant of the LORD with thee: notwithstanding, if there be in me iniquity, slay me thyself; for why shouldest thou bring me to thy father?

These two men had entered into a solemn covenant before God that they would be friends for life, so David pleaded with Jonathan.

He was innocent, he knew that he had done no ill, and therefore he put it to Jonathan, "If I am what thy father thinks me to be, slay me thyself."