• Syria analyst: The regime has created "a humanitarian crisis of its own design"
• Dozens of children have been killed by barrel bombs and other attacks

(CNN)These methods of warfare are so grotesque, they're banned by international law.

Airstrikes on civilian hospitals. Barrel bombs that kill and maim indiscriminately. Chemical attacks that suffocate children with their eyes still open.

But these are the threats Syrian civilians face every day. An estimated 90,000 to 120,000 civilians have been killed since the war began six years ago.

What's the motivation for killing civilians? 

The civil war started after President Bashar al-Assad's regime violently cracked down on peaceful protesters, and ordinary citizens took up arms to try to oust his dynastic regime.

So from the beginning, Assad's enemy was opposition members in his own country. In other words, rebels who had started out as civilians. And that's why civilians are at risk.

"In general, the regime's strategy has been to inflict mass punishment against opposition-supported populations or populations that were perceived to be supportive of the opposition in order to prevent the formation of a viable alternative to the regime," said Chris Kozak, Syria research analyst at the Institute for the Study of War.