To say the MQM is responsible for genocide is a bit far-fetched. I think the correct word Spock is looking for is "ethnic cleansing". The MQM are almost certainly guilty of targeting members of other ethnic groups instigated and more often than not non-instigated. They have driven people away from their homes for speaking the wrong language and having different voting preferences. Not all of these people were thugs, most were innocent people living their lives.
I would also agree that the ANP in Karachi have hooked up with some thugs (thugs other than their political allies - AH or Zardari) and have exploited the Pakhtuns in Karachi issues. Since the mif 1990s to 2007 there was some relative peace in Karachi and quite a bit of development work has taken place. Most people argue the MQM have done 0 for where they have no vote bank. Most pakhtuns were not happy with how resources are distributed hence the formation of the PAC. Thug elements on both sides are exploiting the issue.
What events have led upto the May 12 issue? This was not a sudden explosion. Were the MQM afraid of anyone challenging their power? Why did ANP-PPP supporters carry guns that day if a peaceful demo?