It all depends on how the moment of the murders are depicted.
If you have a Bruce that is just coming back from seeing Zorro, smiling and happy with his parents and you have this nameless thug pop out of no where saying the cliched "hand over all valuables, pal" and a Martha and Thomas Wayne that have pronounced that they've been shot, you just might have a smiling, light hearted, do-gooder Batman. Everyone would have a different reaction, perhaps Batman was just inspired to fight injustice by the death of his parents? It also depends on how Bruce is raised and whether he feels guilt or not. Some times it's by his uncle, sometimes it's by Alfred, some times there's no one. Some times he blames himself for their death, other times he doesn't.
Then again, what if you witness a brutal death. A psychotic, smiling man that shoots your father and mother in the head and you're left alone, in a pool of blood. You probably wouldn't be the same after that. Age has a lot to do with it as well. Sometimes Bruce is 8, sometimes Bruce is 10 or 12 etc. It just depends.
So no, seeing your parents die wouldn't definitely make you a "darker" character, especially if you're too young to really remember. Inspire, definitely, obsessed and filled with vengeance, not always.
Dark like the early comics! Nolan has done an outstanding job of capturing that!
Maybe it's just me, but I don't think Nolan's Batman is "dark" per se. His villains are, but his Bruce Wayne or Batman doesn't really strike me as a "dark" Batman. He certainly isn't light, but he's just not that, you know, typical dark Batman. He's not like the Miller, or Animated Batman, or the Burton Batman.
He's instilled with values and to him, Batman is a temporary thing, a tool he uses to inspire Gotham for good. The Nolan Batman isn't this dark, brooding, vigilante, filled with vengeance.
Going by what I described above, look at how his parents are killed in Nolan's interpretation. Before they're even murdered he's afraid and anxious. Thomas Wayne is always preaching to him about Gotham and goodness. Even when Thomas is laying on the ground dying he's telling Bruce that "it's okay, it's okay". Then, not only does he have Alfred to comfort him, he has Rachel as well.
So no, he's certainly not the grim, dark Batman that most people adore. To be that dark he would have to view the Batman persona as his own, dominant persona. In TDKR it looks like they're going to be moving even farther away from that, even going as far as having him fight out in the day light.