Krita is more of a professional painting tool rather than a pixel art tool.
There are other programs that are available for Mac that are decent to good for making SRB2-like sprites, such as
aseprite (probably the best of the bunch) and
GIMP (not particularly pixel-arty, but you can set brush size to 1 and use palettes), and
there may be more if you also take online tools into account.