The Sides of a Coin
Kenshin has two of the most drastically different dual personalities in manga and anime, and they are quite separate and distinct. He's not just going through "mood swings" when he goes through his Battousai and Kenshin modes; he really does change completely.
The normal Kenshin is a gentle smiling fellow whose sole character defect is that he may be overly polite. Ordinary Kenshin is a person of extreme humility, as evidenced in his speech patterns, such as "...de gozaru..." the suffix "-dono," and the pronoun "sessha" that he uses to refer to himself. This Kenshin even manages to threaten people politely, giving them the option to run away or to -- and here he makes his regret and reluctance clear -- get their asses kicked (but not killed, of course). Kenshin also frequently uses "oro" as a general exclamation which may mean anything from surprise, pleasure, puzzlement, or pain. Kenshin also cannot bear to watch other people get hurt, and will frequently jump into a fight to defend the cowering party. It hardly even matters whose side the person was originally on. If he sees someone strong picking on someone weak, he'll get involved, and he can be shockingly merciless in such cases (well, relatively speaking).
Kenshin's humility also extends to his swordfighting skills and all other aspects of his life. He is never one to brag or boast or show off his knowledge. Unlike most of the other characters in the series, he isn't even easily offended if his abilities are called into question. He answers all of them immediately anyway, in the form of, er, practical demonstrations (i.e. the kicking of asses).
On the other hand, there is nothing cute or polite about Battousai. He is an entirely different person from Himura Kenshin, from the squinty glare and perpetual scowl, to the abrupt rudeness in his speech, and to the clear and explicit threats of violence (...and murder...and dismemberment). Unlike Kenshin, Battousai would not think twice about killing other people. To him, lives are expendable, even his own. Battousai is -- to put it bluntly -- almost insane, in that once he decides to kill someone, it would be very difficult to make him stop. While Kenshin would flinch at the thought of hurting an innocent bystander, Battousai might, if he thought it necessary. Battousai does not say "oro," neither does he ever go swirly-eyed.
Battousai also exudes a sort of "better than you" aura, as evidenced in some of his battles. An extreme example is when he goes to rescue Tomoe and is challenged to a two-on-one battle. His response? Bring it on, it would save the time it would take to track the next one down. Still, he doesn't resort to empty bragging about the size of his sword, etc. As Kenshin or Battousai, he never really has to.
Because of the strange duality at his core, Kenshin tends to see things not as black and white, but in myriad shades of gray. He believes that everyone deserves a second chance (it would be hypocritical of him to think otherwise) and he is certainly capable of disconnecting from his own morals and opinions to see things from his opponents' angle and to understand. One might expect a mighty hero to think he was right all the time, but this is not so in Kenshin's case at all. It is often the opposite: he is usually reluctant to say who was right or wrong in a battle. All one can expect him to say are what he can and cannot stand for out of principle. As a result, Kenshin/ Battousai doesn't have very many "personal" enemies (just that last batch of villains, actually). By that, I mean people who go "I'll kill you because I hate you!" The rest are usually just people with radically different points of views who want to kill him because he and his principles get in their way -- people who think "I'll kill you so that I can rape and pillage Japan!" or "I'll kill you because you're on the other side!" or something like those. Which explains why he can get away with not killing his opponents. Most of them usually find themselves having a change of heart.