The prequel OVA series is 4 episodes, HIGH quality animation. In ep. 1, 2, and 4, Kenshin kills bunches and bunches of people. And you get to see it. In episode 3 it's just talking, and only one guy dies (not thanks to Kenshin.)
The OVAs show his days as a hitokiri (assasin) for the Isshin Shishi, a (blah blah blah political stuff) and he basically kills important people in order to bring about the Meiji era and Imperialist rule during the transitional phase known as Bakumatsu. More importantly it shows his connection to this girl, Tomoe, but meh, I won't spoil stuff. You basically get to watch Kenshin run in and kill people really fast, with lots of blood. The actual plot to it is very sad (unlike the series, which is just sort of goofy with some serious stuff); it runs sort of like a tragedy. But still very cool.
The common theme through the SERIES (this is the 90-something episode series on cartoon network right now) is the bad guys trying to bring out the killer in Kenshin. Some of them get close, but he never actually kills anyone in the series. You only see a few -=flashbacks=- of him chopping down people. If you want Kenshin going rip****, check out the OVAs.
There are also two OVAs that take place AFTER the big, main series. But uh... no point in watching them until after you see the middle.
But if you want, I'd be happy to ruin the whole plot of the OVAs.