Hexxat is my least favorite addition to any of the EE games. As usual, there is only one Dev-approved outcome and anything else leaves you with a pittance of XP and minimal loot.
That being said, the lack of continuity between how you engage with her in SoA and ToB is, like a lot of the new EE content, not quite thought through.

Background:

When my party first encountered Clara in the Copper Coronet, it seemed like she was going to have some sort of quest where you rescued her from evil and then she would join your party and be normal again. You take her into your party, you gear her up, you go off to do her quest. You end up spending a fair amount of time with her, party because she walks so flippin' slowly.

What we got was this bizarrely heartbreaking cut-scene wherein Hexxat murders her in front of you and then turns to you and goes, "Want to join up?"

I actually said out loud to my monitor: "What? NO! You just killed some poor girl right in front of me, you ******!" I normally play as a Chaotic-Neutral character because I think it's the most challenging to maintain balance while also being unpredictable— I went 1000 percent Lawful-Good as a gut reaction and slaughtered her. You get, basically, nothing for that. And on top of that, it isn't even an interesting fight. A few arrows and a sword whack and she dies. No mist, no coffin to stake: Nothin'. The Devs were expecting you to jump for joy as you take this murderer along with you. People seem confused when you don't, because if you do, you get a Bag of Holding. This game makes it very clear that vampires are evil and not good for you, your party or the storyline in any way, shape or form and then the EE Devs 10 years later expect you to wipe your memory and go party with one. If you don't, it's not even worth pursuing the earlier questline unless you're an evil party, of course, in which case this is a very boring role playing opportunity, for you. Except that you don't get the choice to be evil. It's not like you can save Clara or sacrifice her for Hexxat. It's like, "Be evil...or don't get a bag of holding!"

Even the smallest amount of effort from the Devs could have made this a fun quest. There could be two major lines with two options: One is you kill Hexxat and save Clara, getting her in your party and the other is to sacrifice Clara to gain Hexxat, which is fun for evil parties. Because it's a choice. There could also be neutral outcomes where you break Clara off from Hexxat's curse, but it weakens both and you end up with neither in your party. Or whatever. If you, reader, spent more than five minutes on it, I bet you could create something infinitely better than what exists now. This reminds me a lot of Neera/Rasaad/Safana-SoD "choices" which were either A) Do what the Devs want or B) Get nothing. And at least make Hexxat a fight. Even evil parties like slaughtering vampires!

And even then, it's not like Hexxat is this amazing character you just can't live without, so you're willing to forgo roleplaying to get her. There are far more useful cahracters in the game for almost any party build.

The current lameness:

I slaughtered Hexxat, as mentioned previously. Then I roll into the tavern in Amkethran and low and behold, there she is chilling in the back room. When you talk to her, she's like "Hey, it's you." And your responses are "Long time, no see..." She should be like, "Oh, you brutally murdered me after I brutally murdered that girl." And your response should be, "Hey! Didn't I kill you?" or "DIE!" and this time, give us a chance to stake her coffin. So what did I do? Kill her again. Just on principle. To spite the Devs. Why? Because neither time you can kill her does she create a mist that moves toward a coffin. She just dies like an everyday newb. So she shouldn't be coming back to life through vampiric magic. Also, she's a crappy character and her quest is not fun.

Agreed.

Ugh...

Absolutely agreed.

yes its almost like the game is buggy

Are you saying that the things detailed by @snkboarder do not occur in your Beamdog version?

I don't see this as being any worse than a continuity error in Final Fantasy 5. In that game, the first time you stay at a specific inn, one of your party members will go out to the bar in the middle of the night. Later in the game, there is a scene where that same character dies a plot death (no resurrection possible because of JRPG logic, though at least your party members try). If you witness the death before you stay at that particular inn (reasonable because of all the other ways to regain MP in that game), a character who is plot dead (and hence no longer in your party) will be shown going out in the middle of the night.

(There are many other games where you get this continuity error if you use glitches to sequence break or do things that weren't intended to be possible. Even in BG2 Classic, you can kill Imoen in Irenicus's Dungeon by having her read the Scroll of Weakness and casting multiple Contagion spells on her, and she'll still show up, alive, in the cutscene that follow when you leave the dungeon.)

No... I mean it could be but I'm evil and like the Hex romance story so I have not tested killing her out of spite

I agree that she is the most bugged of all the new npc and that is because of her special death state... I don't find her story more or less buggy than some of the other npc options because they all 'push' their own moral code or personal needs

I joined just to pick up a couple of new games and comment on this thread. I don't get why Hexxat comes back if you kill her.

It's game breaking and frankly disrespectful of my choices as a player! Glad to see you guys feel the same way

well 1st of all they all come back because thats the way Bioware wrote their game... if Hex didn't come back that would make her the odd one out

spoiler... a certain bountryhunter is the odd one out but again... because Bioware said so

so all you really are upset about is why Hex comes back [in that location]... maybe you should play her a few times to learn her story?

2nd she is a vamp and even a special type of vamp if you take the time to learn her story so it really not that odd that a vamp can come back after you killed her... makes a hell of a lot more sence than Boo or Viconia coming back imho its not like you staked the bitch last time is it?

to be clear, its not like Beamdog says you must take Hex in your party... you can always just walk past like many new players do... I know lots of players that have never played her

I'm a fan of the old writing. The new writing and content doesn't do it for me. I plan on disabling the EE soon enough.

Actually this is not true. In the original game if you kill a companion in SoA they will not show up in ToB. You can't summon them through the thing because they're dead

the difference between killed and chunked is they still have a body to raise them from... and do you keep their loot

You are correct. I do not know why the other user made untrue and deliberately inaccurate comments.

I think ussnorway was referring to the bridge between BG1 and BG2. It IS true that if you kill off characters like Khalid, Jaheira, Minsc, Quayle, Edwin, Xzar, Montaron et al in BG1, they'll show up again in SoA none the worse for wear. (In fact, you can even say to Xzar "Didn't I kill you?", to which he just says something like "Maybe? A lot of things happen to me that I can't be bothered remembering." XD) You are correct in that characters killed off in SoA do not reappear again in ToB, however.