Can anyone explain? Without bias or anything?
It's weird ... but basically imagine if a German bishop was killed in Jerusalem just because he is a Bishop and because he happened to be there, in the holiest place in Christianity.
It's kind of like the same thing, Mecca is considered the birthplace of Islam, problem is that is on Sunni-controlled Saudi Arabia, there is a huge rivalry between Shias and Sunnis, but they are held together (more or less) by a common believe in both Allah and Islam.
Problem is that a bishop from the Shia community got killed (or executed, depending on which side you take) just because he was criticizing the Sunni-led government, he was taking that political role but the critical part is that he was a Cleric, and killing a cleric in the Islamic world (specially in countries where Islam is the official religion) is kind of like a big deal, like ... A bishop executing another bishop because of political rivalry, it comes down to that in the most basic of the concepts.
People say Al-Nimr was a terrorist and potentially incite ISIS in Saudi Arabia, which is laughable when you consider that ISIS is Sunni, when Al Nimr and Iranian government is Shia, so there is that too.
The UN believes that people from different religions (or in this case Sunni-Shia shared territories) should be able to live within the same area, something as sensitive as Mecca requires a shared (or at least recognized) co-existence between Shia and Sunni, if Sunni oppress the Shia (or reversal in the "alleged" oppression from the Syrian government towards the Sunni population) then a sectarian conflict is created. Also, the Quran mandates that all people should visit Mecca once in their lives, meaning that a Shia-community member should be able to travel there without an existing threat, which the Saudi government just created (or they are defending their interests, again, depending in which side you are on).