For another perspective, let's take the issue of the Gray Guard (CSco). I think that just about everybody would say that they have a much more morally flexible point of view than a typical Paladin. But there are still some lines that even they aren't allowed to cross.

Specifically called out as being unforgivably evil:
- Slaughtering innocents
- Despoiling a temple of your faith

So we have at least two examples of Always Evil acts from a source outside of BoED - and from a class that many people feel makes it too easy to commit evil acts in the service of good. I think that this is a pretty good yardstick to determine where other "unforgivably evil" acts might be. If it's worse than despoiling a temple of your faith, it's evil.