While in the real world, the debate of what constitutes Good and Evil if fraught with gray areas, the Book of Exalted Deeds attempts to set the record straight in a manner consistent and logical to a D&D game and a working knowledge of its truths can resolve much debate within the D&D game. This, in part, is this intent of the book in the first place.
Should Redemption and the imprisonment of evil beings be of interest to your character, then I would suggest reading or re-reading "Mercy, Prisoners, and Redemption" through on page 28 of the Book of Exalted Deeds. For those of you who are curious, these are the same rules I used when Wind converted Shade from Evil to Neutral.
I would also suggest the following for consideration in regard to Violence (Book of Exalted Deeds, page 9). An Exalted creature can, with clear conscience and righteous authority, end the lives of such creatures as:
o Creatures with the Evil subtype. By definition, these creatures
cannot be redeemed. Example exceptions to this truth are so
rare as to occur only once across millennia of the full multiverse.
o Undead. Undead are, in the D&D universe, created by evil
means without exception. Furthermore, the spiritual growth of
an undead creature is permanently stunted in spiritual stasis
and effectively cannot be redeemed. This is true even for intelligent
undead under all but the most extraordinary situations.
o Mindless creatures have no hope for redemption and if they
pose a hazard they can be slain with impunity. This includes
automatons, constructs, oozes, and vermin. A good character
might show kindness to these creatures, but wisely recognizes
when one is a threat to the peace of its surroundings and reacts
o A creature, evil or not, which consistently rejects an offer of
Mercy and persists in its violent or evil actions effectively has
forced the hand of an Exalted character and may be slain
accordingly. However, to be of clear conscience, an Exalted
character must consider whether the creature is compelled or
driven by forces to continue acting in such a manner. A farmer
who relentlessly abuses his neighbors and steals from them
while under threat of his families demise by a local villain is
an example of a creature acting outside of their nature.
You will also find that for some creatures who are Always Evil in alignment that this is typically because certain knowledge or essence of their progenitors evil is passed on to them from conception. An Aboleth is an evil aberration which is born with all the knowledge of all its ancestors. A Tsochar is an aberration formed from the collective conscience of numerous evil Tsochari young. An evil dragon is born with innate knowledge from its parents which slowly awakens within it. Creatures of these circumstances are exceedingly difficult to redeem.
While on this subject, it is clear from various passages in the Book of Exalted Deeds that achieving victory or redemption over evil creatures by the employment of Charm Person, Suggestion, or even Dominate is acceptable and perhaps even desirable. As long as such creatures should be treated as helpless prisoners and not commanded to perform evil acts (pg 10).
To help clarify circumstances, you may always ask for a Sense Motive (or Wisdom if untrained) or a Knowledge check (DC based on the creature type as a standard creature knowledge check) when deciding to what degree a foe is evil. In either case, your character is not merely considering the encounter alone, but the circumstances surrounding the encounter as well as your characters ethos. This allows me, as a DM to confirm one of the following to you:
o That the creature is motivated to act outside normal behavior
by forces it perceives beyond its own power or the creature is
magically compelled (DC 20).
o That in your characters judgement, you know enough about
the creature and situation to kill the creature or pursue an action
without moral or ethical question. In such cases, you can have
your character act with a clear conscience.
o That in your character's judgement, you do not know enough
to clearly decide the creature's fate by such drastic means as
ending its life.