Bereishis (18:25) Be it far from you to do after this manner, to slay the righteous with the wicked; and that the righteous should be as the wicked, be it far from you; Shall not the Judge of all the earth do right?
Eruvin (100b): Even if the Torah hadn't been given it would be possible to learn proper conduct from observing animals. Modesty from the cat, avoidance of theft from ants, avoidance of adultery from the dove and sexual conduct from the rooster.
Rabbeinu Nissim Gaon (Introduction to Talmud): The reason that there is punishment for activities not specifically commanded is because all those Mitzvos which are derived from commonsense are obligated on everyone from the day that G-d first created Adom HaRishon. Not only him but also all his descendants afterwards for all generations.
Rambam (Moreh Nevuchim 3:17): A person is rewarded for any good deed and punished for sin - even though these activities were not commanded by a prophet. This is because a person is held accountable for those things which are dictated by commonsense.
Ramban (Bereishis 6:13): The generation of the flood was punished for theft because it was an obvious sin. Our Sages (Sanhedrin 108a) say that this sin sealed their fate. That is because it was commonsense that it was a sin, there was no need for a prophets to warn them. Furthermore it was evil to both heaven and man.
Rav Yaakov Kaminetsky (Emes L’Yaakov): According to the Halacha, Avraham was not obligated to risk his life to save his nephew Lot…. Avraham risked his life because the Patriarchs were yeshorim . That means that their actions were not governed only by the strict letter of Torah law - but by straight thinking. G d made man inherently yashar . According to uprightness, there was an obligation to try and save Lot… Avraham felt responsible for Lot’s welfare because Lot’s father had died in a furnace because of his belief in the G d of Avraham. Therefore, according to uprightness (menshlikeit) Avraham had to organize his men and pursue after Lot’s captors. In truth the lives of the Patriarchs - which was before the giving of the Torah - was based on the attribute uprightness. This is the meaning of the expression that derech eretz (civility) preceded the Torah… Therefore, this civility and menshlikeit can be expected even from non Jews. Even though they weren’t given all the mitzvos, but everyone can live in accordance with the inherent uprightness - if he wants.
Toras Avraham (Toras HaSeichel HaEnushi #2): … Intellect (seichel) which characterizes man is not simply a means of knowing and understanding data. In fact the intellect is a moral faculty to know what is truly straight. G d created man with this faculty in order to teach him the path of life. Therefore it is obligatory for man to use it to know what is right and wrong and not to distort his straightness and wholesomeness. It enables him to be a moral judge to understand things in full. Thus man is obligated to keep things which he knows are right - as well as that which he has been commanded directly by G d. Both obligation serve to direct him properly…