The direction of the electric field line from the positive to the negative end is determined by the experiment in which a unit coulomb positive is placed next to the charges to determine its magnitude and direction. The electric field direction is always in the same direction as a positive test charged would be pulled or pushed if it were placed within the area surrounding the source charge.
As such, the unit positive charge would repel the negative charge and be attracted to it. If the electric field line was traced, it would go from the positive to negative.
You might ask, “Why not consider -ve charges?”. We can agree to help them find direction, but we only need to pay attention to the negative charge sign.