Kommentar zu Bereschit 50:21

וידבר אל לבם means HE SPOKE words that found ready entrance INTO THEIR HEART: Before you came down hither people spread rumours about me that I was born a slave; through you it became public that I am a free-man by birth. If I were to kill you what would people say? “He saw a party of fine young men and he prided himself on his relationship with them, saying “These are my brothers”, but afterwards he killed them. Have you ever heard of a man killing his brothers?!” (Genesis Rabbah 100:9) Another interpretation is: He said to them, “Ten lights could not extinguish one light; how, then, can one light extinguish ten lights?” (Megillah 16b).

וינחם אותם וידבר אל לבם, “he comforted them and spoke to their heart.” Rashi has this to say about these words of comfort: Joseph demonstrated to the brothers how his own status had been made more secure by their very existence. As long a they had not come to Egypt he had been suspected of being a slave who by a quirk of fate had risen to power. Ever since the arrival of his brothers such an accusation could not be sustained. If he were to revenge himself on them now these same people would conclude that they had not really been his brothers at all but had been hired by him to play that part in order to legitimise himself as a free man. After all, who would believe that he killed his actual brothers!? Moreover, from a logical point of view, if the ten of them had been unable to extinguish his light, how could he single-handedly hope to extinguish their collective lights?

Ten lights cannot extinguish one light... [You might ask: Why did Yoseif compare them to lights? The answer is:] Because the twelve tribes correspond to the twelve stars which illuminate the whole world. Therefore he mentioned to them “ten lights.” I.e., he was saying that despite their plan to kill him, he was saved from them because each tribe is like a constellation in the heavens which cannot be destroyed. And surely he alone is unable to destroy them. But it cannot simply mean that [since ten could not kill him, surely] he cannot kill ten. This would be no proof because Yoseif was a king and could order a thousand people to kill them. (Gur Aryeh)