Could it be that maybe that wall painting at the end is maybe showing that two faction of people fought over certain location which held something precious, probably some sort of power that would give to those who poses it power to rule with the rest.
Now if im holding to that plot it could be that either side succeeded to acquire that location, or few from both sides plotted to get it and rule the others, or someone managed to sneak alone in there during the battle which was fought around, and used or accidentally unsealed something that was on that location. Now i suspect that its possible that in that location was sealed the Titans virus, and that they unsealed it without maybe knowing how it will really work or what it really is, or maybe it just what they wanted. However those who unsealed the virus turned to titans and im thinking there to ones like Ape titan, he is probably one of the first titans around.
Its possible that those who got Titan powers directly from the source got ability to manipulate the "virus' and turn other ppl to titans and control them to fight for them. But since that wall sect in manga is not revealing anything so far since the secret is too dangerous im guessing that when they unsealed or used titan power things went wrong in some way. They probably could control it as they wanted so those who fought around when power was unsealed probably gathered together and escaped and later build the walls which contains titans inside.
Those titans inside the walls were probably ppl who sacrificed themselves to save the rest by building those walls and staying in them for eternity or till the walls collapse.
Now that Ape titan probably has the power to manipulate the titan virus and turn normal people to titans, and i think he did that in Connies village, he turned everyone while others were suspecting that it was a wall breach or titan diggers.
Well that just my idea, something like big picture
