ruler

Assalamu alaykum1.) What should a fair islamic ruler/president/king do for his country and in dealing with the other countries,etc? And what would be his reward for this work? Otherwise what would be the punishment for an unfair Islamic ruler and frankly speaking nearly all Islamic rulers today are bad?2.) Many scholars and many fatwas (even on www.islamonline.net) say that it is haram to try to become a ruler because it is a big responsibility,etc. And this job should come to you and not you for it. And that in the past this job came to many great companions but they refused to take it because of the big responsibility.however I read in many books that there were companions,etc who wanted to become rulers…. Like Umar ibn Abdelaziz. So why then is it haram to work to become a ruler???