User blog comment:Raul123alin/My theory about the origin of the titans/@comment-9096809-20130813071553/@comment-98.196.190.94-20130818063414

The ape titan is the one turning humans into mindless titans.

Hence the reason why Reiner and Bertholdt took down the walls was to prevent the monkey from turning the rest of humanity into titans. They are probably fighting against the titans as well and decide that its best to just wipe them out and prevent the titans numbers from increasing.