Board Thread:Theories/@comment-30785465-20170223183145/@comment-26495049-20170224033856

But on a serious note, I believe that the most likely method the Beast Titan used to turn humans into Titans is poisoning their water supply with a modified version of the titan serum, which turns them into special Titans he alone controls upon his command.

If he could simply turn anyone into a Titan with no strings attached, why didn't he just turn Levi into a titan when the latter attacked him, or even better, turn the entire Survey Corps into Titans the moment they entered Shiganshina?