I am going to make some assumptions and say you are looking for a arch with a 45 deg (or some other angle) miter at each end.

The way you are trying to achieve it is a common beginner modeling mistake. You are trying to do to much in a single step.

First, you cannot extrude the profile curve from an angle because the entire extrusion will then be asymmetrical that is to say skewed.

Second, the angle at the other end of the extrusion will be incorrect. I am assuming that you want the miter to be a mirror of the other side.

So one way to do this is to break it into three steps.

1. extrude the profile normal along the path to create half the arch

2. if using polygons then use cut faces to create the mitered end, if using nurbs then loft a short section between the flush edge of the arch and an angled curve

3. mirror the arch to create the other side.