Why did HaShem send an angel to speak to Avraham?

Question

During the Akeida of Yitzchak, the Torah tells us twice that an Angel called out to Avraham. HaShem spoke to Avraham Avinu many times; why here was an angel used (twice!)?

Answer 1

So the Malbim takes a novel approach:

ויקרא אליו מלאך ה'. הנה הדבור קח נא את בנך היה ע"י ה' בעצמו, והקריאה אל תשלח היה ע"י מלאך ולמה לא עכבו ה' בעצמו. הנה מלבד מה שיתבאר שהיה נסיון גם בזה, י"ל עפ"י מ"ש חז"ל שמכל מצוה יולד מלאך שליח ה' לגמלו טוב בעד המצוה ההיא כמ"ש שכר מצוה מצוה, שהמצוה עצמה תגמלהו שכרו, והנה מצוה הגדולה הזאת נגמרה עתה בשלמות בכל פרטיה ונברא ממנה מלאך ה', אבל אם היה אברהם שוחט את יצחק, אז היה נחשב כרוצח, כי לא היה רצון ה' רק שיעלהו לא שישחטהו, וע"כ קרא אליו מלאך הזה בעצמו לעכבו מן המעשה שבה תלוי מציאותו וקיומו, וע"כ אמר במד' שהמלאכים בכו כשרצה אברהם לשחוט את יצחק, רצו שהמלאכים שנבראו מן המצוה הזאת שהיה בה כמה חלקים, הקיחה וההליכה ובנין המזבח ועריכת העצים והעקדה והעליה על המזבח, שמכ"א נברא מלאך, וחיות המלאך וקיומו הוא מן המצוה הזאת, ואם היה שוחט אותו היו כל המעשים הקודמים הכנה אל רצח ותועבת ה', וע"כ בכו מלאכים אלה כי יהיו כאין וכאפס

And an angel of Hashem called to him - The request to 'take now your son..." came through Hashem Himself, and yet the calling out to not send (his hand i.e. to kill) was done through an angel. Why didn't Hashem Himself delay it? (I.e. why the need for the angel if the original command came directly from Hashem). It must be that that there was a test in this also. One can answer as per Chazal, that out of every mitzvah (command) an angel of G-d is born to reward him well for that mitzvah, like it writes, the reward for performing a mitzvah is another mitzvah (Pirkei Avos 4:2) i.e. that the mitzvah itself will reward him for his reward. And here this great mitzvah has now ended in its entirety in all its details and an angel of G-d was created from it. But if Avraham had slaughtered Yitzchak, he would then be considered a murderer, because it was not the will of G-d; (Hashem's intention was) only for him to ascend and not to be slaughtered. And so this angel himself called upon him to prevent him from the act on which his reality and existence depended. Therefore, it says in the Midrash that the angels wept when Abraham wanted to slaughter Isaac, the angels wanted that he (this angel) be created from this commandment which had several parts - the taking and the going, the building of the altar, the arranging of the wood, the binding, and the ascending to the altar - each action created the angel, and the life of the angel and his existence is from this mitzvah, and if he (Avraham) had slaughtered him (Yitzchak), all the earlier actions would have been preparation for murder and abomination of G-d . Therefore, these angels wept that there would be absolutely nothing.

As far as the subsequent, second calling:

1. Radak writes that the angel was now informing him that as compensation for what he had just been prepared to do, Hashem would likewise compensate his children when the occasion would arise to do so.

2. Alternatively, Chizkuni asserts that the angel was acting as an intermediary for G-d and he was given permission to speak on behalf of Hashem.

3. The Tzror Hamor notes that since the angel had been involved first time round, he returned a second time to speak with him. He posits that Avraham at that point must have been wondering over the turn of events, i.e. that he had come incredibly close to killing his son and that he had been commanded to sacrifice his son, only for nothing to then actually happen. He was therefore carefully weighing everything up, deliberating if he had acted in the correct manner. Thus the angel came a second time to put his mind to rest.