"Simplistic" is not the same as "simple".
The Kach political agenda of the 80s was from straight out of the Tenach: "1.There is One G-d. 2.That G-d is the G-d of Israel. 3.The Jew has G-d given ownership over all of Eretz Yisrael. 4.Gentiles who don't tow the line will be expelled. 5.Immediate death penalty for terrorism. 6.The immediate dismantling of the Histadrut. 7.Income tax to be abolished. VAT will be 25%. 8.Compulsory ban on all transportation on the Sabbath"
Compared to the agendas of other parties, Sefardim loved it, as did any Ashkenaz worthy of his salt. Kach was on line to get 15 Knesset seats in 1988.
The Kach political agenda is Torah-driven, true. You have the right to think that the Tanach is "simplistic", although I believe that it is more a superficial understanding of the Torah that makes it look simplistic.
Besides, there is no need to be a religious person to support Meir Kahane's ideas. All that is needed is to care for historical truth and fairness, and for the survival of the Jewish people. That is the reason why there are many secular supporters of HaYamin HaAmiti, for instance. I don't think that the "secular" support for Meir Kahane's agenda is based on simplistic ideas either. On the contrary, it is based on historical knowledge and critical analysis of the myths and lies of the widespread pro-Arab propaganda.