It is part of larian's design philosophy, what they call N+2. Quests will have the "intended" solution, a fallback solution if the player does X (be it kill someone important or sell something they needed) and a second fallback for D:OS 2 if another player completes part of the quest in a competitive environment.
Allowing players to kill anyone gives them freedom to play how they want. This will be particularly important in D:OS 2 with potential PvP questing, as you can kill the NPC player 2 is trying to help, to accomplish your goals. It is more organic and immersive to not put arbatry limits on who can be attacked. The only limit is are you strong enough.
Ultimately as has been said before, the more freedom you give the player the more they can be immersed in the game. (no sudden message saying you can't do that.) But there will be consequences, in this case a player min maxing the game to get the most XP possible by killing everyone and everything.
At the end of the day should they not be allowed to do that if they find it fun? The game should be balanced around the average or a bit above average XP gain to not force someone to min max, but they should still have that option. Increasing the non-combat XP options would be a welcome change but it should not come tied with a reduction to current options.
