Every path in RK has cost. And I doubt that most people will agree on you that RK is not mature. The thing is that blood, gore, and killing alone, is not what makes a series mature. However, the main argument of this, would be, is Rurouni Kenshin AS material. And it undoubtedly is. Explain to me how RK is less mature than Bleach, which has somehow found its way onto AS. And then there is Trigun. That show has a very optimistic view for the most part, but that was also aired on AS. And look at Shin-Chan. That show is made for children in Japan, yet it is aired on AS. And what about the manga, did you read the entire series for it ? You are saying that Death Note is mature, and I agree, but have you ever considered how many things in it are highly unrealisitc, and in a sense, childish. For example, there is no way that one man, could ever gain the entire support of the World's Police alone, even for a second. Also, many things such as wealth and money that are used to house the facilities in which people like L and Near were stationed, were very ill-explained, and some of their reasoning was farfetched. My point in all of this, is that if you're going to be all nit-picky with Ruouni Kenshin, you may as well take this into consideration, every single anime, in one way or another, will have an aspect of it that can be viewed as poor, or immature, or childish, even ones that people claim to be a stroke of geneous, such as NGE (I haven't watched very much of it to make a judgement, but I have noticed how unrealistic some of the characters inthe show,act). Anyways, all you're looking at his Kenshin's view of life, which seems to be your only basis for calling RK a less mature series. That, is something that I have to disagree with.
i am getting tired of paragraph form so i'll just seperate my response into categories:
maturity: first, i am not being "nit-picky" with Rurouni kenshin, I believe that it shows little or no cost for Kenshin's path (from what I have seen he is the only one hurt by it through physical injuries he would have probably gotten anyways). In trigun there is a much more even distribution of pros and cons for Vash's path. The other thing is that the mood is way too silly and happy (even compared to trigun) to maintain the maturity of other shows. Even in bleach the tone is almost never as upbeat or silly. This is a very big part of the maturity of a show. In Rurouni kenshin the social interactions and comedy often are at the level of childrens cartoons in their lack of individualy based aspects. On shows like bleach there are unique ways in which characters interact on an individual level. But in Rurouni kenshin the characters are quite limited leading to vast generalizations in character. For example, many of the opinions in the show seem to be set as general facts and as such characters will all respond in the same way. This puts the character relations into a simpler, easier to manage form. This doesn't happen in most adult swim shows were the characters always have their own intrests and perspectives in mind while they talk, making it less preachy and more mature.
Adult swim material: First of, i would have to disagreewith what you said about adult swim not having good shows. In fact, there have only been two in their anime lineup that have been poorly reviewed. Additionally, as i have said, Rurouni Kenshin has far worse production values than anything they have had recently. The animation is okay, but most of what gets on adult swim is increadible either artistically or technically. Also, i have heard the voice acting in the dub and it is very unprofessional when compared to the modern dubs. Just compare it side by side with Samurai Champloo, Paranoia Agent, or Eureka Seven and you'll see what i mean. And though you might think otherwise, the animation is extremely important when you cater to an adult audience, especialy one that is skeptic about anime. Anyways, i don't even think that depth is much of an issue for adult swim broadcasting. There are plenty of series far more deep than Rurouni Kenshin that haven't gone on adult swim. The point is that in order to be successfull, adult swim has to focus on the shows that are the best in many areas and in it's time Ruruoni Kenshin was not nearly up to standards set by other shows.