it's starting to sound like exalted feats are WAY more trouble then they are worth.
Ding ding ding! We have a winner!
That sentence describes a lot of the Book of Exalted Deeds. In many cases it's going to be more trouble than it's worth for players and the DM.
Granted, the rules thankfully do add for leeway in various cases. For example, the Lawful Neutral Succubus Kerrus mentioned would not be a "kill on sight" kind of encounter. There is a picture in the BoED on page 7 showing a paladin with her sword drawn against a succubus and an incubus with the caption "A paladin must choose between destroying evil and honoring love." The fact that this is a choice says that succubi are not "kill on sight" regardless of their alignment.
Note that there is a section on mercy also starting on page 7 of the BoED:
"MERCY
For good characters who devote their lives to hunting and exterminating the forces of evil, evil’s most seductive lure may be the
abandonment of mercy. Mercy means giving quarter to enemies
who surrender and treating criminals and prisoners with compassion and even kindness. It is, in effect, the good doctrine of
respect for life taken to its logical extreme—respecting and
honoring even the life of one’s enemy. In a world full of enemies
who show no respect for life whatsoever, it can be extremely
tempting to treat foes as they have treated others, to exact
revenge for slain comrades and innocents, to offer no quarter
and become merciless.
A good character must not succumb to that trap. Good characters must offer mercy and accept surrender no matter how
many times villains might betray that kindness or escape from
captivity to continue their evil deeds. If a foe surrenders, a good
character is bound to accept the surrender, bind the prisoner,
and treat him as kindly as possible. (See Mercy, Prisoners, and
Redemption in Chapter 2 for more about the proper treatment
of prisoners.)"
In general, it’s a good idea for the DM to make sure that the
players aren’t punished unnecessarily for showing mercy to
opponents. If every prisoner schemes to betray the party and
later escapes from prison, the players quickly come to realize
that showing mercy simply isn’t worth it. It’s fine for these frustrations to arise once in a while, but if they happen every time,
the players will rightly give up in frustration.
There are other rules going over forgiveness, bringing hope, and even redemption. The paladin who finds the incubus and succubus together in the coils of love might see that there's a glint of hope for them to be redeemed, and going after that spark is certainly worth sparing their lives.
Another quote from the BoED, page 9, this time regarding violence:
"In fact, even
launching a war upon a nearby tribe of evil orcs is not necessarily good if the attack comes without provocation—the mere
existence of evil orcs is not a just cause for war against them, if
the orcs have been causing no harm."
The mere existence of something evil is not a just cause for using violence against that evil.