[ccooffee]

How is Twitter tied into this problem?

The first reporting on the topic was the Al Jazeera English tweet, which used "killed". The second reporting of the topic was the Al Jazeera (Arabic) tweet which included the video and the Arabic word for victim/casualty.

We don't actually know what sources various reports actually used. I would not strongly fault a reporter who found both the AJ English and Arabic tweets and trusted the translation. This story hinges on the difference between "500 killed" and "500 casualties", which is a distinction that many native English speakers aren't good at picking up. I would absolutely believe that a reporter who passed the AJ Arabic tweet through a machine translator would assume the computer made a mistake by choosing "500 victims" when compared to an ostensibly expert-translation from a different media outlet.