Can a person be a great leader and a decent human being at the same time?

Great question.  It's not politics.  You have that war against politics to fight.

Yes, of course.

At first.  But if they are in power too long they are bound to become corrupted eventually.

YES................

people who dedicate their lives to assuming power and money are usually terrible people.

the few good people who find themselves in power are going to go bad too,at least to some extent.

It depends on the kind of leader. World leader in charge of a nation that actually matters? No. Leader of some kind of charitable organization? Yes.