Why do I fail quests I haven't accepted or started?

How can I fail quests if I haven't started them yet, or even accepted them?

For example I went to prison to save the new sheriff for Primm, and while killing the Powder Gangers I got the message that I have failed the quest "I fought the law". Same thing happened again while cleaning Vault 3, I killed a fiend named Motor Runner while helping NCR and same thing happened, I failed some other quest apparently related to him. Last example, I went to Freeside and entered the laser gun store (can't remember the name now) and all of sudden everyone started shooting at me! Also I failed some quest.

Can anyone explain me why this happens? I can't access Gamefaqs, so I can't read articles in there.

Solution 1:

It's because you have killed off what I'll call "objective markers" for future quests. What I mean is if there is a quest related to you interacting with a person, and you kill that person before the quest starts / you complete it, then you fail in that particular quest - regardless of whether you knew about it beforehand.

For Motor Runner there are a couple of different quests such as helping the NCR and a drug related quest for the Khans. The quests are as follows

Aba Daba Honeymoon

Bounty Killer (the one the OP was most probably doing)

Killing him off will fail the latter of the two in your case.