OK, so there's beef. Where is the motive for the hijacking though? They didn't make any demands (yet) or used it for an attack. What is gained by Iran for stealing a Malaysian airliner other than to use it as a weapon or steal its cargo?
The possible reason could be to 'teach a lesson' or put pressure on Malaysian Govt. The rift between Sunni and Shia is beyond an airliner or its cargo. If you ponder over their 1400 years old history.
So for 1400 years of bloody, hatefilled history they steal a plane full of Chinese citizens to teach Malaysia a lesson? I'm probably just ignorant, but I'd have to guess that that level of risk isn't worth blind rage without furthering some agenda.