Halo 2 had a pseudo-3D display detailing where people killed and were killed from, including lines drawn to match up the killer with the killed. There's not many examples of it online anymore, but I did find this one that has a screenshot:
http://www.hanselman.com/blog/Halo2...veYouSeenTheGameViewerAndTheGamesRSSFeed.aspx
Might be overkill for what you're trying to do, but if you're recording the x,y,z of both people along with the weapon used, it's certainly within the realm of possibility.
A simpler solution might be two heatmaps.. one image of the location of every person when they kill an enemy, and a second to record deaths. That way people can flip between the two images to get a bit of the context. Additional images breaking down each weapon type might be nice also.
Going back to Halo 2, the problem with their heatmaps is that as more games were logged, the entire map turned into a giant blob of red. Probably won't happen with TF2 in this day and age because of the smaller player counts but it is worth considering.