What were Abraham’s motivations in your eyes? He could have resisted Sarah’s entreaties to have a child by Hagar.
I don't know his motives. But I know from a thorough reading of the Bible, that God didn't tell him to do so.
Perhaps he wanted to help God along with the plan for him to have a son. Perhaps he reasoned (as Sarai did,) that she was too old. Perhaps Abraham jumped at the chance to sleep with the sweet young thing.
I do know that God can and did use the sins of Abraham and me to His good. That is a promise from the Bible...everything (even evil) is used for God's good purpose. I trust him in that promise. I put my faith in the divine providence of God Himself, in all his wisdom, love, justice and might. Abraham is God's servant and only God can judge him. Only God can judge you or me.
Thank God!