Im affraid this isnt bug ...
If you take Gale with you, he eventualy tells you that if something happens to him (meaning he die) and he stays that way, aproximately after 3 days his "magical orb" he carry, will erupt and most likely kill everything in quite serious radius.
And that is exactly what happened here.
It's poorly conveyed (hopefully, because this is EA), but this is the intended result for killing Gale because of his current condition.
I would still say this is a bug. I killed Gale and left him right where you first meet him. I did NOT have him join my party. I had no interaction with him other than clicking the "attack" dialogue option when I was forced to talk with him.
Immediately it had him give me the speech about how to resurrect him and acted like we were in a party. However, this is not the case.
I never had him join my party. He wasn't bound to me in any way.
