I'm a new Krita user, and trying to throw together an animation to see how to do so and how time-consuming it is. I threw together a sprite for a warrior and a sword for it. (Never mind that the sword is several times bigger than the warrior, that's intentional.) Then I took the Transform tool, set it to the Box Filter, rotated the sword into place, and...oh, that doesn't look right. I'm not sure how to fix that...I tried for a bit, but couldn't make it look right.
A different tool gets me more promising results, but the filter doesn't seem to transfer. Which is a shame; the way that tool displays stuff makes it much easier to precisely place the sword such that it renders into something usable.

Is there any way to use the second display thing with the first tool, or the first filter with the second? Or some other way to rotate that sword in a way that's simple to clean up afterwards?

You're working with a very low resolution image and that will have very visible consequences when you rotate an object so that a clean vertical/horizontal line becomes a diagonal line.

When you rotate the sword, the transform tool rotates the sword image at the screen resolution (or close to it) to let you make fine adjustments with good visual feedback. When you finalise the transform, the end result is then converted to the image resolution, with interpolation filtering, with obvious consequences.

Box filtering is intended to maintain pixelised appearances so you might want to try one of the other filters such as Bicubic to give a smoother appearance to the edges. You can undo the rotation and then do it again using a different filter to see if that gives a better result. If you perform repeated rotations on the sword, you'll probabably get increased smearing at each stage so it would be a good idea to work on a copy of the original sword image at each stage.

The second tool you used is the Warp transform. This has it's own form of internal filtering for final image formation that is fixed and not available for adjustment. Using a warp transform to rotate a simple object is an amazing use of that tool.

When you do the animation, I recommend that you use animated GIF as the rendered output format, to preserve what little resolution that the image has.

I thought I'd already posted a reply/clarification, bu tit turns out I haven't, so...here goes.

I'm aware that it's going to be a rough, jagged edge. It's pixelart, it's supposed to look like that! The problem comes when the rotated image doesn't have straight-looking edges. You can see what I'd like in the penultimate image I linked, where the pixelated image is...well, I can't say what you guys think, but I think it looks nicer. It would be nice if there was a way to get that kind of result without trial-and-erroring my way through the Transform tool or just making every frame in Paint.net.