By Rosally Saltsman

According to the Midrash [mondelinge overlevering], the Jews had an ample supply of water and in order to stay alive, the Egyptians had to buy their water from the Jews. This brings up an interesting point. If the plague lasted seven days, and the Egyptians had no water to drink for that time, might they not all have died if they had not gotten their water from the Jews? That being the case, could the Jews not have gone out to freedom after one week of the first plague? So why didn't they? All they had to do was not sell their Egyptian taskmasters water. But they couldn't do that. One of the traits of the Jewish people is rachmanus - mercy. They may have wanted to withhold water from Pharaoh and his soldiers, but they didn't have the heart of stone to withhold it from young children, old women and nursing mothers. The Egyptians probably knew whom to send to arouse the pity of the Jews and so they were able to obtain water. A person who does a mitzvah with a particular object is always helped by that object. That is perhaps why G-d made sure that water flowed freely for the Jews in the desert and split the sea for them to escape their pursuers, because they didn't let the Egyptians die of thirst. Doing kindness with friends and acquaintances is expected of everyone. Doing kindness to enemies is something only the Jewish people are famous for. 

It is noteworthy that before seven of the ten plagues, Hashem warned Pharaoh through Moshe and Aharon of what would transpire. One reason for this is to show that it did not come about by chance, but that Hashem planned it and informed Pharaoh exactly what was going to happen. Another way of interpreting the warnings is that it is a kindness to warn someone of what may befall them, so that they may take heed and avoid it. Even if someone has to inflict pain on someone, be it a parent or a bank manager, a teacher or a law enforcement agent, forewarned is forearmed and the severity of a blow is always lessened when one is prepared for it. So not only did Hashem show chesed with Pharaoh when he warned him of the plague, giving him a chance to repent, but his warning allowed those who believed in Him to prepare themselves psychologically and physically for what was to come. Issuing a warning before taking drastic action not only softens the blow, it may avert the need for it in the first place.

From Rosally Saltsman's book, A Portion of Kindness.