We have to understand Moshe's plea of "ושמעו מצרים" i.e. that killing the Jews will ruin Hashem's good name because the Goyim will say that He lacks the power to save them. That is not Moshe's business but Hashem's. If Hashem's reputation will be harmed among the Goyim then it is up to Hashem to redress that wrong. But who is Moshe to bring up an issue that pertains to the good name of Hashem? If Hashem is OK with it [and of course Hashem already thought of it before Moshe brought it up] then Moshe should ALSO be OK with it!?
Therefore we must explain as follows: In Parshas Ki Tisa after the חטא העגל it says
"Now let me and my anger will flare on them and I will destroy them and make you a great nation".
Rashi there explains that we don't find the Moshe davened yet Hashem says "let me". So this was Hashem's way of opening and opening [פתח לו פתח] and hinting to Moshe that it is up to him and if he davens then he can save the Jews. We see that without the "פתח" that Hashem opened for Moshe he would not have been allowed to daven. So here in our context how can Moshe daven without Hashem giving him an opening??
From this it would seem that when it says "ושמעו מצרים" it is not a reason not to kill the Jews but rather opening to the tfilla of Moshe. Regarding the Makkos and miracles and strong hand against Egypt, the pasuk says
"וְיָדְעוּ מִצְרַיִם כִּי אֲנִי יְהוָה בִּנְטֹתִי אֶת יָדִי עַל מִצְרָיִם וְהוֹצֵאתִי אֶת בְּנֵי יִשְׂרָאֵל מִתּוֹכָם".
In other words the Makkos were administered to teach the Egyptians that Hashem is G-d. This message would be lost if the Jews are killed now. So it was the assurance that the Egyptians would learn the lesson that Hashem is G-d that opened a pathway for Moshe to daven now "מבלתי יכולת" - If You don't save the Jews it will be attributed to Your lack of power which will obfuscate Your earlier message!! So this was permission for Moshe to daven just like "הניחה לי".
