Weird Bevelling

You could try some of the bevel methods. Switch them around and see if it changes your result. Also when you feel ready, remember to try and move the pole out of the corner. Also maybe the viewport shading is making things look a bit weird, try changing the matcap,

I think Omar has the answer. It's also a bit hard to see what's happening to that bottom vertex. How that's connected or if its edges have bevel weights will also influence the shape of the bevel.