Is it unethical for a professor to lie to their students?

Yes.

Is is still unethical if the professor argues that the lies were for the students' own good?

Yes.

Is it still unethical if, a few months into the course, the professor tells the truth?

Yes.

Is it still unethical if, a few months later, the professor tells the truth and one or more students says "you know, I understand now why you lied to us"?

Yes.

This really isn't a challenging conundrum. The student-teacher relationship is a special one. There are expectations that need to be fulfilled on both sides. One of the most basic ones that can be expected of a teacher is don't lie to your fucking students.