SureAI

Are all quest npcs essential (immortal)?

I am asking because some bear

[+]

came from the street and stolen my vodka

came to Giliad and slayed few npcs including Brother Patrick and some beggar and someone else and also few chikens (must be lucky bear since "it" was with full health when I spotted all this mess).

Won't I miss any quests because of that?

Quest NPC's are mainly essential, but some merchants and trainers that are outside the buildings are not essential and no respawning (like brother Patrick), so you can miss some rare item.
You can check here:
http://nehrim.wikia.com/wiki/People#B

You can always resurrect them by console.

Thanks for a tip. I already viewed nehrim wiki, just asked to be sure.
As far as I know brother Patrick does not sell anything good (only some rare ingredient for low alchemy skill level invisibility potion which may be need early in game on hardest difficulty). But I'll leave it dead. I think it more crashproof in future that way.

Did you already get the teleport rune for Giliad?

What I'm wondering about is why almost all NPCs in the Abbey are essentials, even the shepherd is.

I like it that way. All quest npcs essential + all quest interiors locked until you need to enter there for quest.

Not every Quest related Interior is locked, if you don't have the quest.
Actually there are only few Interiors that are locked and these few are mostly mainquest related.
I hate running through the same dungeon twice when you got the quest after you were already there but if you would lock every quest related dungeon there wouldn't be enough left to scavenge.