I'm not Maliki's biggest fan but holding him solely culpable isn't entirely fair. Yes, his policies have tended to favour the Shia majority (wrongly), but he's not this great dictatorial figure his opponents depict him to be. He couldn't even pass an emergency powers act in parliament to deal with ISIS because of how many parliamentarians voted against him. Not even the country's biggest Shia factions are fond of him - he's constantly clashing with the likes of the Sadrists and Badr brigade.
Absolutely.
What's amazing is now Muqtada Al-Sadr is being pictured by many as a "moderate shia" because he is opposing Maliki, and, delibertely or not, forgetting that Muqtada Al-Sadr was actually behind the terrible sectarian violence that errupted in 2006. Go back a few years and look what everybody was saying about Muqtada Al-Sadr being a terrible Shia extremist who is responsible for the death of all the Sunnis.
What happened eventually is that Al-Maliki didn't want any militias outside the army, which is why he fought A-Mahdi Militias and practically destroyed their influence in Baghdad and the South (he launched a major offensive against Al-Mahdi militias in Basra in 2008 and the army stayed there for years to ensure that there are no more Shia militias there).
He also put many leaders of Shia militias (who had a role in 2006 sectarian war) in Baghdad and the South in prison.
The other Shia party who was accused of sectarian killing and torture was the Islamic Supreme Counsel. Baqir Jabr Al-Zubaidi (who is a member of their party) was the interior minister at some point (if some here still remember him) and he was also accused of mistreating the prisoners, sectarian crimes...etc. Now the Islamic Supreme Councel is also against Al-Maliki.
What's ironic is that even Al-Sadr and Al-Hakeem (leader of Islamic Supreme Counsel) are now accusing Al-Maliki of being "sectarian"! Shut up I would say.
The main differences with Al-Maliki are political really, but they hide behind that "sectarian" excuse because it provides them with more leverage to apply more pressure on him (both popular and international). Kind of, in some way, like the Republicans calling Obama a "Muslim".
Al-Maliki is not a perfect PM, and he made a lot of mistakes, but all the other parties who are accusing him of being "sectarian" are actually far more sectarian than him.
And like you said, look at the parliament. He can't make any decisions if he doesn't have the approval of the other constituents. How can he become a dictator that way? Look at the Iraqi forces who fled. Many of the leaders are Baathists (and many of them are Sunnis) who didn't have any loyalty for him or for the country. If he was a dictator and he's ruling for 8 years now, don't you think he would have built far more solid and loyal army forces??