Question

Is work done by a non-conservative force always negative

Answer 1

A non conservative force is a force, where if the starting and ending positions are the same, the work can still be negative.

Friction is an example of a non conservative force.
If we push a heavy box in circular path, the starting position and the ending position are the same.
but the whole time friction was resisting the motion. This is why work done by friction is always negative. Because it always works in the opposite direction of motion.