The Or HaChaim (Bereishis 20:9) writes, טובה תחת רעה כמשלם בעולם רע מתחייב אדם שאין, "No one deserves punishment like one who repays a favor with evil..." When the Yidden left Mitzrayim they carried arms, as it states (Shemos 13:18), מצרים מארץ ישראל בני עלו וחמושים, "Bnei Yisrael were armed when they went up from the land of Mitzrayim." The Chasam Sofer (Toras Moshe, Beshalach, וחמושים) asks, if they carried weapons, they could have used their weapons and battled the Egyptians who pursued them! Why did they need the miracle that the sea split?
The Chasam Sofer explains that 210 years earlier the Egyptians did a great favor for the Jewish nation. At that time, there was a famine, and Mitzrayim opened their doors for Yaakov and his offspring. Therefore, the Yidden had hakaras hatov for the Egyptians. Raising arms against a nation that once welcomed them would be improper.
This vort shows us the extent of the obligation never to harm someone who helped you. The Mitzrim afflicted and enslaved the Yidden. The favor the Mitzrim did for the Yidden was so long ago! Nevertheless, the Torah obligated them to have hakaras hatov. It would be wrong to use weapons on Mitzrayim, a nation that once did them a favor. How cautious we must be with the obligation of having hakaras hatov!
Rashi (Devarim 23:8) writes, הדחק בשעת אכסניא לכם שהיו ,טעם מה ,ליאור זכוריכם שזרקו פ"אע ,וכל מכל מצרי תתעב לא, "Do not completely reject the Mitzrim, although they threw your first-borns into the Nile. Why? Because they provided you with shelter in your time of need."
