It says in the Mechilta on the verse Exodus 21:14:
"And when a man plans to kill his neighbor through deviousness, [the murderer] you will take him from my Altar to die" - A neighbor, meaning 'others'. Isi ben Akiva says that before the reception of the Torah we were warned about murdering another. And, after the reception of the Torah these restrictions were lessened. In truth, we became exempt from the laws of life and death. (Not to mean that we were permitted to kill. Rather, this is simply a way of interpretting this verse for the sake of exegesis)It can be understood namely that from the word "neighbor" or "other" we learn that the killing of a non-Jew does not warrant the death penalty. Then, the words of Isi ben Akiva present us with two contrasting states - the state of "before the reception of the Torah" where we were bound within the confines of the laws given to the offspring of Noah - and the state of "after the reception of the Torah". The Marchevet HaMishna comments on the words of the Mechilta:
"And, after the reception of the Torah these restrictions were lessened? - That's a surprise!"That is, can we say that it's possible that once the Holy One Blessed is He gave us the Torah and all is commandments, which included commandments that were previously given to us, suddenly that which was previously prohibited (such as killing a non-Jew) is now permitted? Thus, in our own time was are warned about killing a non-Jew (it is prohibited).
According to this the continuation of the Mechilta "In truth, we became exempt..." is understood: Although it is forbidden for a Jew to kill a non-Jew just as a non-Jew is forbidden to kill a non-Jew - just as it was before the reception of the Torah - there is a slight change for the case of a Jew killing a non-Jew: A non-Jew is given the death penalty for killing another non-Jew, but a Jew isn't given the death penalty by the court[for killing a non-Jew]. This is because after the reception of the Torah a Jew is already obligated to follow the 613 commandments in the Torah, and the killing of someone who is bound by the 613 commandments- this has to be an act that warrants a punishment with a special stringency. The source found for the Seven Commandments given to the Offspring of Noah and their accompanying punishments don't suffice. And therefore when a Jew kills a non-Jew he is not given the death penalty by the court. Rather, the Jew is killed at the "hands of Heaven."
Furthermore, we learn from the words of Isi ben Akiva in Mechilta that the prohibition of a Jew killing a non-Jew is in virtue of the prohibition of killing given to the offspring of Noah. Jews are also included in this prohibition. However, we were given a special addition to this prohibition, "Do Not Murder", which relates only to the killing of a Jew. And thus, a few warrant the death penalty.