It seems like the Titans have a really huge advantage like the colossal, armored, female Titan. They could probably charge at Wall Sina instantly, take it down and wipe out humanity. The female Titan spared Armin's life a few times, when she could take him out in seconds.
So it seems the Titans are capable of killing all the humans completely but they don't. Why is that so?
This has no answer yet, and probably will not for a while. The author has stated the publication will run for at least another three years from the date of the linked article (September 2014).
Below is what has been revealed so far:
The reason seems to be that the Coordinate, which has the power to control Titans, is "lost". Mindless titans can't be controlled without it.
It could be said that Colossal Titan only emerged recently to take down the wall since a Titan of that size and power has never been available before.
Titans are actually used to enslave/control the population (since the royal family seemed to have had the Coordinate power for several generations). They never attempted to use it against the "enemy" before Eren took control of it, which is odd. Bertolt, Reiner, Annie could actually be working for the king, since the royal family "lost" the Coordinate to Eren's father.
The exact answer has not been revealed, but some conclusion can be drawn from different episodes and chapters of anime and manga.
The prelude of several episodes of Attack on Titan stated that:
Over a hundred years ago, humanity suddenly found itself faced with a new predator. They were more powerful than humans. Humanity was immediately pushed to the brink of extinction. The survivors built three walls: Maria, Rose, and Sina.
From the anime and manga:
- The Female Titan spared Armin's life several times.
- The walls were built with the exoskeletons of titans.
- Colossal Titan, Armored Titan, Female Titan (i.e. Bertolt Hoover, Reiner Braun, Annie Leonhart respectively) and other Titan Shifters actually lived among humans.
- Ymir turned into titan and protected her friends from other titans.
- Rich people and the top brass who mainly lived inside Wall Sina knew a lot about the titans, the walls and the titan inside the walls.
- The titans who broke through the wall and led other titans inside were mainly the Colossal Titan and Armored Titan, and their objective was to search for the mysterious power known as the "Coordinate".
These points lead me to conclude that their main intention is not to destroy humanity, but to find the Coordinate ability for which they may destroy humanity. The relation of the Titan Shifters with human, and their willingness to sacrifice their lives for their friends shows that their main intention was not to destroy humanity. And humans are not as weak as they seem to be; titans need to make some effort to destroy all humans, and those titans with brain know that.
The Titans can't wipe out the human race by themselves. They couldn't break through the wall for over 100 years, and still required the help of the intelligent Titan Shifter to break through the wall at all.
I think certain Titan Shifters are on a kind of a mission — maybe they want to show the human race that they can't sit around and do nothing? Or perhaps they have an even bigger goal?
I read somewhere (sadly I can't remember where) that the Titans stopped their mission of attacking because they are afraid of the new Titan Eren. He could be a problem if he finds out the truth behind the Titans.
