By default Slicer shows standard axial, sagittal, coronal oriented slices. If your volume is not axis-aligned and the spacing between slices is very large then you may get staircase appearance.
For non-axis-aligned acquisitions, you may want to show the slices in their native orientation, by using “Rotate to volume plane” function.
If you want to process a volume that has highly anisotropic spacing (e.g., 10x larger spacing between slices than spacing between pixels within a slice) and you want to process it (segment, register, etc.) then it is strongly recommended to crop&resample the volume to have isotropic spacing, using Crop volume module.