I'll have to mildly agree with you on that Tycell (and I say mildly because it is hard to fully agree with someone who spells their handle in "1337"). It would be more fun if you couldn't complete a quest not critical to the game, so you would have to go back and play it again to complete all the quests you learned about beforehand. Although, if you could not complete a quest in the game due to a programming bug, that's just not fair.
POTENTIAL SPOILER BELOW
There are benefits to not killing Renesco. These benefits can only be taken advantage of during the same dialogue screen right after you agree to help him. Once you close the screen, the benefits are gone. I can't say whether this works in the 1.00 or 1.05 version, I've only tried it in the 1.02 version.