if you decide to use the measure of ethical leadership you could propose and empirically test the effect of high or low ethical leadership, but not unethical leadership (e.g., low level of ethical leadership is positively related to....)
You may use the unethical leadership instead of the ethical leadership questionnaire. I would recommend reading the following artilce:
- Karianne Kalshoven , Deanne N. Den Hartog , Annebel H.B. De Hoogh (2011) Ethical leadership at work questionnaire (ELW): Development and validation of a multidimensional measure, The Leadership Quarterly, Vol. 22, pp. 51-69. doi:10.1016/j.leaqua.2010.12.007.