Currently the player can take actions (such as killing Spencer) which would
render the game unable to be won.

The game should warn the player if such a state occurs.

I think that the best course of action is to make the game be always finishable.

Certainly that would be the best course of action.

But until we do so (which might take some time) I think we ought to indicate
that the game cannot be won.

Of the top of my head we ought consider the death of the following bots/dialogs
as making the game un-winnable:
HF-EntranceBot
Kevin
KevinGuard
Spencer
WillGapes

This is not correct - the death of e.g. KevinGuard is not a problem after it has
opened the door for you.
Same for Spencer once he gave you the last quest, and so on.

I object to a patch that claims "the game is now unwinnable" when it's not true.

Certainly.

What is needed is a boolean (e.g. int essential_to_win) added to the npc struct
(src/struct.h), that has a default of FALSE (added to enemy_new() in
src/enemy.c), updated in dialog (src/lua.c, and dialogs), and checked on npc
death (func. kill_enemy() in src/enemy.c).

I might even write a patch to do it.*


* More complicated behavior, such as a dialog asking if the player wants to go
back to a previous (winnable) save can be added later.

Patch at http://rb.freedroid.org/r/1818/

Actually, never-mind about that -- apparently there is already support for lua
code executing on the death of an NPC, it just had never been used.

It probably should be made so it isn't so level-specific, but, that isn't
required for this use case.

Committed in c97558ad078398c36d7f5b102bc445d4eb8c480c. Thanks :)