Basically, I am wondering if it is possible to beat this game without killing anybody in combat.

Or is there some point where the game forces you to fight and kill enemies?

The very first quest is to kill a zombie to get his key. I believe there is no other way of obtaining it other than killing it. However, I believe you can do the rest of the game without killing anyone. Mostly running away when the situation heats up and ignoring some of the quests where you have to kill someone cos they simply attack you whatever you say. I havent finished the game yet.

No. Quite sure that no. At some point of the game, when you going out to the new areas, there are a few really tough fights, which seems to be unavoidable. I believe most of fights with people may be avoided by diplomacy and fights with other creatures - by running out, indeed. But such strategy closes one of the most important sources of experience points, so at some point your character would be not strong enough to handle the challenges...

Even if there is some proof that it is possible, I don't think it is possible to be accomplished by a casual player, who don't want to spend an hours of preparations...

Almost.

There are 4 mandatory & unavoidable fights in the entire game, one of them is to kill a zombie at the very beginning of the game. Rest can be avoided/escaped.

I remember that I tried that back in the days. The only one I can remember I had to kill was the zombie at the beginning. Maybe it was more but that is the only one I remember you HAD to kill. I think you can avoid fighting all the bosses though.

I should try doing a (near) pacifist run again :)

A completely pacifist run would be difficult is not impossible, but most violence can be easily avoided.

Don't know if anything is changed in the GOG version of the game, but I managed to acquire the key from the zombie by examining them until I found one clutching key - then in dialogue I had an option to rip the key from zombie's hand. They did not become violent and I ended up with the key without having to kill anyone...

I m not sure if anything is changed for other fights.

EDIT: No, I m not right - you have to kill the zombie at the beginning. I've probably suppressed that bad, bad memory. ;)

haha yeah it's definitely an obligatory

"this is how you combat" tutuorial type mechanic that also dictates that NO, you will not pacifist run this game

I think I did a fully pacifist run back in the days (not counting killing the zombie in the beginning, which I think is mandatory)

You need to fight that angel, one of the incarnations, Ravel,and zombie at the beginning.
And I'm not sure what happens if you have none as companion, but I suspect you have to make that fight either way.

There's no need to fight the Incarnation(s) if you qualify for the checks (either from actions ingame and/or stats). But you'll have to fight one of the two companions either way.

It can be avoided if you cheat a little.
With the tome of cheating you can change your class to thief and steal the key, but you have to save and try again and again because if the zombie "recognize" yout try you still have to fight it.
But without cheating it's impossible... Sorry.