Ambassadors are sent to other countries to build relationships. They are sent to hostile regions to get the sides talking to each other. They do so by finding some common ground amongst the parties, and then building upon that common ground.
Imagine how a conversation might go:
"I'm Chris Stevens, ambassador from the United States. I'm pleased to meet you."
Over time, conversation would turn to family and what life is like in the West. It would be a way for the parties to understand: 1) what experience each is coming from, and 2) their ability to relate to the experiences of the others.
When Ambassador Stevens says he is not married and has no children, no gilfriends, no tales of dating to share with the others in a moment of "guy talk," what would you expect the presumptions to be?
Moving away from this topic to the larger issues, I believe he was likely assassinated because he was good at his job. Given that he was fluent in the language, and was willing to be secreted into the country on a cargo ship, likely working from safe-houses with cardboard boxes as office equipment, I'm sure he earned the trust of the parties he negotiated with. I'm guessing that he was too successful at getting the parties to at least be willing to talk to one another first. This, by itself, could be seen as a threat to al Qaeda, who would rather see the parties shooting at each other instead of talking to each other.
That made Steven a threat and a target. There likely wasn't a deep bench behind Stevens to fill the void if he were taken out. That made him a valuable target to disrupt progress. Taking out Stevens would set back gains in the region and force diplomacy to start over.
-PJ
Stevens assisted in the overthrow of Gaddafi, helping arm the very radical muzlims who butchered him. So I don’t know what you mean by “effective”.