We're just not as evolved as some people think we are.
If we look at the animal kingdom, there's a one main reason why animals of the same species kill/attack eachother, power. This can be divided in several sub-reasons, which are (& I might miss a few).
1. The right to mate
2. Territory
3. Prevent other's offspring to reach adulthood
We humans have added a few sub-reasons, but it's still about power.
1. The right to mate (remember the Trojan wars?)
2. Territory (speaks for itself)
3. To kill other's offspring (don't think we still do that on purpose, but it might still happen subconsiensely (SP?))
4. Economical/financial power
5. religious power (although religion is often just used as an excuse)
As you can see, thanks to the fact that we've evolved a bit further than most animals, there are more things that are precious to us (I won't see a chimp fight over a lump of gold... I can however see two humans fight over it).
In general, we fight because we want to stay in (or get) power, just like any other animal & unless we become aware that we're acting like animals & want to change that, nothing will ever change, except maybe survival rate of the human being, since I've never seen a chimp fight with guns & nuclear weapons (another improvement that we could only have achieved because of evolution).
This case is no different. The religious leaders in Afghanistan see this man as a threat, because more people might follow his example (even though it is unlikely) & they'll lose their power base. The only option is to eliminate him.
Of course we all disagree with it, but we're not the people in power over there (in Afghanistan).
