Qara is far more evil than Bishop, with less reason. She literally goes orgasmic at burning stuff (revealed in one of the conversations).

Bishop is merely bitter and petulant; he doesn't destroy to amuse himself. During the last game, I used the cheat code to "correct" her alignment, and will probably do that again. This way, she can use Shar's belt and some other evilly-aligned gear.
If you want to play an evil spellcaster, look to Qara for some ideas. Startear and Ammon are evil warlocks, but their evilness is mostly revealed in absolute self-centeredness. Ammon is a destructive bastard: he plows ahead without thinking or asking because he and he alone can and will do what must be done, without the hesitation of morality. Is he more evil than Qara?
I simply can't play evil worth beans, unless it is acting (which includes live role-playing). Evil characters get all the good lines, all the advantages, and the best scenery to chew on.
In this game, I enjoy playing a chaotic good drow. Last time I was lawful good, but being chaotic is more fun. If you release the imps at the Academy, they'll appear in the stands during the Trail by Combat to cheer you on. People don't expect a compassionate drow, but there are some amusing conversational tags, and it makes recruiting Kistrel and dealing with demons more natural. The CON hit is hard, but the INT boost is welcome. Being level-adjusted is a pain, but the natural spell resistance is worth it. It all balances.
However, I could recommend that you try a Drow Arcane Trickster as an evil character. If you start with an INT of 20 and first level Rogue, you get oodles (56?) of skill points... this means that you have plenty to put into conversational skills (you'll need Bluff as well as Diplomacy or Intimidation). Pickpocketing will get you some extra weapons, etc., to help survival, and there are places in the game where you will have to deal with locks and traps. Put points in UMD so that you can use non-arcane scrolls and gear. I've done fine in 4 games with a STR of 8. You can park gear in safe places or on characters that aren't being used. Casavir wouldn't be a fit companion for an evil drow, so treat him like a mule-headed, um, mule. (You
are evil, remember?)

After 2 levels of Rogue (to get to BAB 1), you can start adding some wizard levels.
I prefer wizard to sorcerer. Sorcerers can blast away, but their spell progression is slower and they are limited to picking spells at level-up.
Wizards can scribe spells into their spellbook at any time! If they are of the right level, they can have all the spells of that level.
Most arcane spells can be scribed onto a blank scroll. I've had difficulties scribing Meteor Storm and Undeath to Death, and suspect bugginess. I'll have extra copies of certain spells (such as anything that kills Undead or Outsiders) so that if I'm in an area where resting is not allowed, I'll still have an arsenal.
Certain spells such as Fireburst are very effective. If you are going to have aggro because you are a sniveling little rogue spellcaster, use it to draw the enemy within toasting range, and then toast them en masse. I haven't tried feinting or hiding in plain sight, but a devious player might like playing "gotcha!'
Many times during the game if you ask for or expect a reward, you take an evil hit. I'm not sure why (it is suitable payment for services rendered, at personal risk and cost). If you aren't trying to be goody-good, you can collect these rewards. There are a couple of vaults and buildings that you can only enter if you are on the evil path. The Warden of the Keep gets a special short sword and there are other rewards on that route; I doubt if the designers had something unique for Dread Lord of the Keep, but you ought to at least get some interesting dialogue choices. Hmmm... if there is a quest on the dragon's mountain for an evil character to become good, what about something suitable for a paladin who is lured into selfishness by the sight of all that treasure?

At least one thing: you won't have to sit through that really terrible ending with the flat-voiced narrator!
Oh, your foster-father looks around the Keep and says that you've done well, but if you rescue the drow merchant, and you are a drow, she comments upon all the slaves who serve your whim. Dread Lords do need their minions!