In BG1, You can't kill Gorion in Candlekeep (insta death).
I'd love to know how you think the game should proceed if you manage to kill him (and the insta-death was removed) - short of creating a completely new set of main quests 
A couple of things on my mind.
1. CHARNAME should be able to escape Candlekeep one way or another. The fact that you can't leave through the front door is due to railroading and wanting you to go with Gorion. Adding an exit to the front of Candlekeep is appropriate and one back into Candlekeep is too. The front door could be locked. CHARNAME could use open locks, steal the key from the guard, charm the guard, or kill the guard.
CHARNAME could leave Candlekeep before talking to Gorion. Gorion would catch up with CHARNAME and could continue on with CHARNAME.
2. People have long wanted to be able to help Gorion as well. I suggest a following modded Change. Create a New Area that is a copy of the Area where Sarevok ambushes Gorion and CHARNAME. the sides of the area encompass only what is seen there (aka smaller).
After Sarevok says "I'm sorry that you feel that way old man" and Gorion says "Run child, get out of here!", CHARNAME gets the Dialogue and gets a chance to choose. The options are
A. Run
B. Fight
If A is chosen, then CHARNAME runs away and the auto battle cutscene happens as normal.
If B is chosen then AI Scripts take over the fight. The scripts are written to match the cutscene as best as possible. CHARNAME can join the fight and risk death and potentially kill Sarevok. Enemies will prioritize Gorion and then CHARNAME. At any time, CHARNAME can go to the edge of the map and click the scroll wheel to exit.
IF CHARNAME exits, then the Chapter One intro "The dawn is especially cruel this morning" will fire off.
If CHARNAME fights and dies, then Game Over as usual.
If Sarevok is killed, then we get the Sarevok Death Movie and a modified version of the below unused String
Lost Epilogue-Text (chptxt8.2da)
String-Ref 20631
The struggles you have faced are over for now, and you shall climb from this vile place victorious. Your actions here will ease the burdens of many, although most will never know the name of their benefactor. Nonetheless, the citizens of the Sword Coast can rest soundly this night, even as you lay troubled in whatever inn you find yourself. Sarevok's death is not the closure you had hoped for, and many questions remain unanswered. There will be more trials to come, but they are for another day... Such is your lot, child of Bhaal.
Then we'll get a cutscene of a new Scholar, Irenicus, making a visit to Candlekeep and CHARNAME finding himself/herself drugged. Irenicus will say "Sleep Now". Then we transition to BG2 and CHARNAME wakes up in Irenicus's duengon.
Perhaps upon being amgushed by Sarevok and Crew, CHARNAME could be
3. If CHARNAME kills Gorion in Candlekeep. CHARNAME could stay or leave. When leaving Candlekeep, we do the same mini area as above.with Sarevok ambushing just CHARNAME now.
Sarevok's text will be different.and will try to ascertain who CHARNAME is. CHARNAME can...
A. Tell the truth
B. Lie (Good Charisma Needed)
C. Run Away
D. Fight
If CHARNAME avoids or runs away from a Fight with Sarevok and crew, then the game proceeds as normal with text modified to reflect that Sarevok is hunting CHARNAME but that Sarevok did not kill Gorion. References in BG2 should be accordingly modified if CHARNAME and not Sarevok killed Gorion.
If CHARNAME fights and defeats Sarevok, cue Sarevok's death cinematic as above and have Jon Irenicus show up early as above.