Aid workers are a target in Afghanistan. There have been 300 attacks this year and over 30 killed. Often, they are seen as helping or spying for the occupiers. Whether that is true or not, NGOs are in danger. Christian Aid (or the other relief agencies) were not invited to go to Afghanistan, they chose to go there.
In this latest case, the woman was killed because she worked for a foreign NGO. When it was reported she worked for Christian Aid, the Taleban (who are claiming responsibility) then said it was because she was a christian. There is no evidence that when she was killed, they knew she was a Christian, and it is more likely that they only found out themselves from press reports.
Needless to say, we didn't make a fuss when it was
Koreans or
Japanese or
Germans being killed, but for some reason we prefer to believe the Taleban's pathetic excuses in this case.