Or HaChaim on Genesis 42:7/אור החיים על בראשית מ״ב:ז׳

1 וירא יוסף את אחיו, Joseph saw his brothers, etc. When he saw them he felt as their brother and displayed friendliness towards them. It was only from the brothers' side that he appeared as a stranger. The Torah emphasises that he acted like this because the brothers did not recognise him. Joseph could speak to them sternly without the brothers taking offence as brothers. After all, he was a stranger to them. Joseph's whole purpose was to devise a scheme whereby also Benjamin would come to Egypt. He also wanted to test them to see how they felt now about what had transpired between them many years ago. He then became aware that they were sorry for what they had done to him and considered themselves as having committed a sin against him.