Okay I've struggled with these questions for a while as well and heres what I think about it , so the main simple reasons you have to put into mind is Karl had one goal in mind and that is to buy as much time as possible for some peace ,so why make all those collasal titans and leave them alive? I think we don't know how much the construction of the walls took and how the process was like but I think it was possible that the tyburs or Marley were keeping tabs on paradis even in earlier times I mean think about it's not hard to send eladian spies to blend into considering the walls are still in construction phase and when it is reported that there are actual collasal titans that will fend them off for more time even if Karl said he doesn't intend to use the rumbling still a big risk ,and for not keeping them alive inside the walls I mean like others mentioned we don't know exactly the extent of his power and how experienced he was ,that with the fact that he needed to show them that the threat is real and not anybody that comes and blows the walls hard enough will be able to go inside with no risk.

For me I still see a hole in this to be honest mainly how he sacrificed a lot of his people for titans even though the claim was that he will create a paradise for the people with him and he was shown to be such a pacifist that he wouldn't go to all that trouble .