This basically comes down to the fact that I (and many others) completed 'The Replicated Man' without getting what is arguably the best weapon in the game. Harkness is useless now, and yet, he cannot be killed.

I can understand making it impossible to kill NPCs that are vital to quests (because of people complaining that they ruined their game) but is there any reason (other than laziness from a programming and game-design standpoint) that these NPCs cannot be killed and looted AFTER all quests they are required for have been completed (or failed)?

No, there is no reason. As a customer, I would greatly appreciate it, at the very least, if a patch were released to allow Harkness to be killed (and looted for the rifle) after having failed or completed 'The Replicated Man.' I would even more-so appreciate it if this change were made for all NPCs (or especially if this game-play element were made an integral part of Skyrim). After all, creating a realistic and believable game-world is what you guys are trying to do, isn't it? One where NPCs simply go "unconscious" after being hit with a nuclear bomb, is, well, stupid.

As a side note, this is my first time on the forum, I'd like to add that this website is beautiful and very well designed. Kudos to the person(s) responsible.

Can't you steal it for him?

from him I mean.

You cannot. Nor is the rifle contained in any containers. Of course, pickpocketing doesn't give access to all items an NPC has, the armor they're wearing being an example of such.

They [bethesda] obviously wanted to make the rifle a reward for choosing the good path. unfortunately, you can do the good path without ever running into harkness (and getting the rifle from him). i found zimmer because the storyline brought me right to him, and i lied to him to save harkness' ass, ending the quest.