Halo is like Twitter with 60 characters instead of 140. Very hard to communicate in server on complicated rules, so thanks for bringing it here.
There is no rule covering slaying per se. You can obviously kill as long as you're racing. What you can't do is
1) Camp. Can't be making kills when you could be racing.
2) Chase kill. Can't pass up a hog, or go away from your nav (on foot or in a hog) in order to make a kill.
If you are on foot, and there are no hogs, you can kill any nearby enemies. Depending on the map, they may spawn at the other base and you'll have a better chance of grabbing a hog when it spawns or getting into a teammate's hog.
Obviously there are gray areas. If you decide to kill an enemy it takes a few seconds to finish him off, so you would pause briefly in your racing to do that. Sounds like what he did was probably ok as long as he did it quickly right after spawning.
Of course if you're attacked, you can defend yourself, and you can stop racing to do that.