This is almost impossible to answer without ruffling someone's feathers but here is some food for thought.
Based on the idea that you are making reference to one of the ten statements (עשרת הדברות) and there is a teaching that all the 613 mitzvot are contained and alluded to in the ten statements, (See the commentary Heichal HaBracha of the Komarna Rebbe on the giving of the Torah.) the concept mentioned in the Mechilta on this is worth considering.
There is a question about what the Jewish people actually heard from HaShem directly. The view of Rabbi Yishmael is that they heard the first two statements which include all the others. They responded 'Yes' to the positive commandments and 'No' to the negative commandments. That is because they arethe general categories of positive and negative encompass all the commandments.
But the opinion of Rabbi Akiva is that they heard only the first statement directly, which includes all the others. They answered 'Yes' for the positive commandments and 'Yes' for the negative commandments. Here's a link: http://hebrewbooks.org/pdfpager.aspx?req=40606&st=&pgnum=88
This view of Rabbi Akiva parallels the order set down in creation mentioned in Bereshit. The 10 statements at the giving of the Torah parallel the 10 utterances of the creation of the universe.
Based on the language we say in Lecha Dodi, "Shamor v'zachor b'dibbor echod", Negative(Negative and positive in one statement, G-d caused us to hear.), it seems the view of Rabbi Akiva is accepted.
In this context, the commandment of 'You shall not murder' is a subset of the first commandment and would therefore not be 'the most important'.
Another interesting perspective is found in Sefer Avkat Rochel by Rabbeinu Makir ben Abba Mori, book one, chapter one, which says the Sages say the last of the ten, 'Lo tachmod', is the greatest and all the others are dependent upon it. It goes on to say that one who fulfills 'lo tachmod' is considered to have fulfilled the entire Torah. Here is a link:
http://hebrewbooks.org/pdfpager.aspx?req=42999&st=&pgnum=3
Here again, it is emphasizing the concept of all the others being contained within the one.
In the context of Avkat Rochel, this appears to be emphasizing the kabbalistic concept of 'Ohr Yashar', emanation from above to below and 'Ohr Chozer', the return to the source from below to above. In either case, the point of origin contains all the particulars that follow it.