Eventually of course he is recruited by the guild of assassins that feature so prominently in the trailers. In a nice change of pace from other movies of this ilk they do _not_ train him in a montage. They spend quite a bit of time both on teaching him the techniques and trying to convince him that being an assassin is a good idea. Unfortunately at least 75% of this convincing consists of variations on telling him that he needs to grow up and be a man like his father, not like the wimpy office worker he was before. The other 25% of this is showing him how they are the "assassins of fate" and by taking out their designated targets they could potentially be saving thousands of other lives.
So the main character is fairly well developed, although by "well" i mean they spent a fair bit of time at it, not that it was handled particularly subtly. After about 30-45 minutes into the film you've got a still somewhat naive but bad-ass killer who thinks he's working for truth and justice and fuzzy bunnies or something. Which leaves a significant amount of time left for further plot development... (And they did pull off at least one interesting plot twist that i didn't see coming.)
So there are kick-ass superpowered assassins, there's lots of action and lots of bullet effects and obviously lots of killing. I believe that everyone we actually see getting killed onscreen is either an assassin, a knowing employee of the assassins, or sneering corporate fat cats who, as a member of the middle class, i can easily imagine having done something that they deserve to be shot for :)
There was at least one situation where a significant number of innocent bystanders were clearly killed, though we don't actually get to see the deaths. However throughout the entire movie i actually felt sorriest for some rats which were booby-trapped, which is probably a minor spoiler but i think it's important to let those who like rats be forewarned.
The amount of blood and gore is "moderate" for the amount of violence that occurs. However as that level of violence includes people getting shot in the head frequently, (er, but not individuals who are shot in the head frequently. Frequently shot in the head only once? :) there certainly is a fair amount of blood. There's no exploding heads or other body parts or spurting arteries, but there are exit wounds (occasionally significant ones) and a "realistic" amount of blood spray and splatter.
shelleycat actually thought they understated the amount of blood somewhat, whereas i was of the opinion that it just seemed that way because Hollywood so frequently overdoes it to an absurd degree. In any case, it clearly wasn't particularly sanitized, but neither was it over the top gruesome.
So anyways, if after this you've decided you don't want to see the movie because you either like rats or dislike blood, you ought to go check out this Wanted in 15 Minutes post :)