Immediately it had him give me the speech about how to resurrect him and acted like we were in a party.
Thats quite some interpretation ...
I for one would simply say that his spell was "preprogramed" to simply apel to listener self preservation.
Yes i understand that your Evil, murdering oathbreaker thinked "why should i bother with this shit" and moved on ... but that was (as it shows up) pretty unwise decision.
"Actions have consequences" makes sense if you were bound to him in any way. Unfortunately just killing him doesn't mean that he should be able to wipe out an entire team in 3 days.
Is that so?
Lets imagine he have a TIMED bomb planted in himself ... big, strong, scarry, magical bomb ... wich it basicaly is ... ok?
Now, you killed him, okey?
But did nothing about the bomb, bcs you didnt even know about its existence, nor about its principle.
Then his Minor Image warned you that something reeeeeeeeealy bad is going to happen, unless he is ressurected ...
But since you felt no desire to help him, you ignored this warning.
Got it?
Great.
Now tell me please ... why exactly do you expect that bomb to stop ticking? O_o
Bcs thats not how timed bombs usualy work.

And if this magic that he has is so powerful that it could just kill anyone in 3 days, why aren't any of the enemies using it?
3 reasons actualy:
1) It dont kill "anyone" it kills "everyone" ... including the wielder, including the person who would want to use it as a weapon (most likely) ... wich is hardly worth it.
But i can imagine special division of Tactical Nuclear Gale in some funny hollyday special comics.

2) They dont have any ... this "orb" (as Gale calls it), is in fact part of original soul of Godess of Magic herself ... its not something you would simply buy in nearby shop.

3) Its quite possible that nobody except Gale knows about this thing.
The fact that it acts like you're in his party when you kill him is a bug.
Is he tho?

---
it is rumored that some of the companions, probably pretty much the ones who are not actively in your party, will die at the end of Act 1 anyway, which means that we will be confronted with the problem of Gale as a living bomb again...
That ... or this alone prooves that this rumor is nothing but that, just a rumor.
