Why is the electric field negative?
Electric field is not negative. It is a vector and thus has negative and positive directions. An electron being negatively charged experiences a force against the direction of the field. For a positive charge, the force is along the field.
Electric fields generated by electric charges originate from positive charges and terminate from negative charges. An electric field exerts on electric force on any charge (other than the charge that produced it) in it's vicinity.
An electron experiences a force against the field and a positive charge along the field.