How to Play a Good-Aligned Character

[Cazero]

Almost all the laws you approve of will align with gut feeling.

And that gut feeling aligning in enough people to end up being proclaimed as a law is no coincidence. There must be some reasons for it, and figuring those reasons would help understanding morality better, wich helps making Good choices.
So the question "why is poison Evil" is not only legitimate, but also crucial. Especialy in the context of the [when stabbing someone in the face isn't] addendum.

Yes, you will eventualy get stuck at a prime proposition that can't be justified (like "harm is bad"). But "murdering someone in their sleep is just bad" is nowhere near that point.