Greatest Principle of the Torah - What is it?

In my research concerning empathy and the Torah - I have come across some interesting sources. The following is evidence that the famous statement of loving your neighbor as yourself - is not viewed as empathy. The question is why is this emphasized in Shabbos (31a) where the potential convert says, "Convert me on the condition that you teach me the entire Torah while I stand on one foot. Even though it is in the negative form of "that which is hateful to you don't do it to others." It still seems that this principle is ultimately rejected even by Rabbi Akiva - after the death of his students.

In addition the Abarbanel asks if this is such a critical principle why isn't it one of the Ten Commandments?

Bereishis Rabba(24:7): … Ben Azzai said, “This is the Book of the descendants of Adam” is a great principle of the Torah. However Rabbi Akiva said, “You shall love your neighbor as yourself” is even a greater principle of the Torah. [It is only] because of [Ben Azzai’s] principle a person shouldn’t say that “since I have been been embarrassed, my neighbor should also be embarrassed with me. Just as I have been debased, my neighbor should also be debased with me.” Rav Tanchuma said, “If you do so, you should know whom you have embarrassed because man has been made in the likeness of G‑d.”

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בראשית רבה (כד:ז): ...בן עזאי אומר זה ספר תולדות אדם זה כלל גדול בתורה, ר"ע אומר (ויקרא יט:יח) ואהבת לרעך כמוך, זה כלל גדול בתורה, שלא תאמר הואיל ונתבזיתי יתבזה חבירי עמי הואיל ונתקללתי יתקלל חבירי עמי, א"ר תנחומא אם עשית כן דע למי אתה מבזה, בדמות אלהים עשה אותו.

I have in fact found the Torah basis for empathy - but more at a later time.