Here are some of the Sunni/Shia' issues from my own perspective.
Well, as far as we know that thses issues are the most complicated issues in Islam..
I'll write here some few information about this mess..
~One of the issues here is: Who's going to be a Caliph after the death of Muhammed Sunnis say: Muhammed chose Abu Bakr as a Caliph.. Shia say: Muhammed chose Ali as a Caliph..
~ Another issue: the hatred turned worse at time of the Caliph Uthman ibn Affan... Sunnis say: that this problem was made up by a Jewish man named Abdullah Ibn Saba, who felt that it was a good opportunity to accomplish their aims of rebellion by starting arguments over religion. While Shia say: that this problem was made up by a Jewish man named Ka'ab al-Ahbar for the same reason too.
~Again, the battle of Jamal as a result of Caliph Uthman's murder.
Here is video of a moderate Shia cleric trying to mitigate the hatred between Shi'a and Sunni.. In this video he was talking about the battle of Jamal...I'm not sure if he's telling the truth or not!! Since I'm from a Sunni background, they told us not to trust them *because of the (Taqyyia) thing* Haha! :P
All what I know is that cleric is trying to mitigate the hatred and condemning those other Shia' clerics who insult the companions of the prophet Muhammad..
~Note that insulting the companions of the prophet is considered a blasphemy in Sunnism~
Those Shia muslims who are influenced by their clerics who insult the companions of Muhammad, are involved in that sectarian violence and vice versa.
~Note That both moderate Shia and Sunni clerics are being attacked by fundamentalists muslims from both two sects! Just for what they saying, and just for being moderates~
*Another issue: Which is all these conflicts that occurred at the time of the Caliph Ali ibn Abi Talib and after his death..There was a political dispute between Ali and Muawiyah which lead to many battles and blood shade..
Ali ibn Abi Talib was murdered and then, and his son Hussein ibn Ali was about to be set as a Caliph but died at the battle of Karbala..
I have tried to summarize that story as much as I can!
And yes! Homo sapiens are terrible species!