Why does my big armor print sag on one side near the end?

That lean near the top is almost never about the print being top heavy. What's ussually happening is that as the stack gets taller, the whole thing acts like a lever on the plate, so any tiny spot where the first layers aren't gripped well starts to get tugged with every pass of the head. By the time you're up near the top corner, that wiggle shows up as a lean or droop. Check whether your plate was really clean before the print, and whether the corner that leans sits closest to the edge of the build area where adhesion is often weaker.

Tilting it back 10-15 degrees helped me on a similar big flat piece. It moves the overhang on that droopy corner into a spot where supports can actually reach it, and the layers aren't stacking perfectly flat on each other anymore. Worth running a couple test strips at the same angle first before you commit to the full print, since a big tilted footprint can pull up at the edges if your bed isn't level.

Came here to say the tilt worked for me too. I did a big flat shoulder pauldron tilted back around the same range and the corner that used to curl came out dead straight. One thing I'd add is to give it a wide brim, because the first time I tried the angle the front edge of the footprint lifted about halfway through and I lost the whole print. Second run with the brim held fine and the seam where I would have glued was never there at all, so the seamless look is definately doable.