Noach 
 
The Ramban (6;13) explains that even though there were no commandments during the generation of Noach, Hashem nevertheless decreed that the world be destroyed because of robbery. The reasoning for that is because basic human logic dictates that one shouldn’t steal. Therefore, even though the nation of Israel weren’t actually commanded not to steal-they still knew that they were doing wrong by not stealing. However, the Rambam states that a non-jew who fulfills the 7 Noahide laws because they seem logical won’t merit a portion in the World to Come and isn’t even considered a wise person. However, if so, how could Hashem kill the whole world for stealing? They were never commanded to not steal and weren’t supposed to do it simply based on logic!?! 
 
Additionally, the Rambam (Moreh Nevuchim) states that until Moshe, no prophet ever commanded people to perform mitzvot. Therefore, we know for sure that they were never commanded not to steal-so how could the world be destroyed for sinning? How could Hashem destroy the whole world for a sin in which He never commanded them not to do?!? 
 
Furthermore, the Gemara (Sanhedrin, 58b) stayes that a non jew who doesn’t do any work on Shabbat is liable to the death penalty. The Rambam, however, states that even though he is liable to the death penalty he is nevertheleas not killed. However, how could one be liable for death and yet not be killed? 
 
Lastly, why exactly didn’t Hashem command them to perform the mitzvot? 
 
Rabbi Yochanan Zweig explains that the 7 Noahide laws give soceity a right to kill a person. Every society has an obligation to set up an environment where people know the laws and that know that they are obligated to keep them. We therefore see that when one gets punished for sinning–the punishment comes from society’s obligation to kill someone if they don’t observe the laws. Meaning, it’s the court that gives punishment when one sins and there must therefore be a society in order for one to receive punishment. 
 
Rabbi Zweig explains that the purpose of the Noahide laws are to perfect society–not individuals. As a result, until a person hurts soceiey by not keeping the laws they cannot be punished. Based on this we could understand why the Rambam states that a non-jew who doesn’t work on Shabbat is liable to death and yet he isn’t killed as a non-jew doesn’t hurt society by keeping Shabbat. Meaning, a person is only killed when they actually hurt society. Therefore, even though the Gemara (Sanhedrin, 58b) rules that a non-jew who doesn’t work on Shabbat is liable to the death penalty, the Rambam nevertheless states that he isn’t killed because in order for the court to rule that one should be killed it must be that they caused some sort of damage to society (which isn’t the case when a non-jew observes all the laws of Shabbat).