Li Shenron
Legend
We have a no evil PC rule at our table.
That is the short form of it. It is really a 2 part rule:
1. No outright intra-party conflict. The party is a team, characters can disagree and have tension but cannot attack, steal from, sabotage, etc. other PCs.
2. No brazenly evil acts. This one is like porn, can't define it but we know what it is when we see it.
-- Example 1: Usually the PCs have a choice of plot hooks to follow to end up in different adventures. One of these hooks happens to be an old tomb that is rumoured to have treasure buried with its occupant. Robbing the tomb is desecrating it. This is an evil act, but it's not brazenly evil. Some PCs in the party argue that the dead aren't using it and if they had it they could accomplish more, so it is for the greater good.
-- Example 2: A merchant has something the PC wants so the PC kills the merchant. This one is just flat out not allowed. It's just not fun for our group and allows 1 PC in the party to dictate the course of the game.
Excellent house rules.
I don't actually enforce alignment on the PC, so I would allow your example 2, but it's obviously an evil behaviour.
The problem you highlight with rule 2 also has something to do with the increased voyeurism of the media in the last 30 years, and how it has affected culture and morals (just notice the % of movies or series that seem to be required to show intercourse scenes, when it is already perfectly clear that character X and Y are having a relationship). So to me it makes a huge difference whether the player of an evil PC says "I am going to kill the whole town (period)" or instead indulge in the detailed description of how he kills one bad guy. I would allow the former even if it's a greater evil act, but disallow the latter (the description, not the act).