Can a tenured professor (who intellectually bullies the students in his/her lab) lose tenure? Are great sins forgiven when you have tenure?

I have no idea what you mean by intellectually bullying. It is extremely hard for a university to fire a tenured professor. Being hard on students or being eccentric is not grounds for dismissal even for a lecturer. Universities don't judge people on "sins." That is more the purview of the church. Universities will deal with accusations of academic or sexual misconduct or anything thing that breaks a law. Telling a student they aren't any good at something is none of those things.

no

and if he is reported to the dean , he can loose his job

Probably not. I know one like that. He had very negative comments on Ratemyprofessor.com and threatened to sue them to get the comments taken down. There has to be more evidence of misbehavior by tenured faculty than complaints from students. Most complaints are of the "he doesn't praise me even when I'm doing everything wrong, and I'm not happy" variety.

Intellectually bullies? Sorry, never heard that term, can you give examples.

But a tenured teacher has a lot of leeway in teaching and research, but obviously great sins are not forgiven. Any violation of university rules can get a tenured teacher fired.

Unless this teacher shows a documented and repeated violation of mistreatment of students, then they can not be fired. If you and others feel mistreated it should certainly be documented. Perhaps one report doesn't mean much, but if reports keep getting made then something will eventually get done.