Iran didn't shoot down that airliner on purpose. They put out a full report. They should have grounded all airplanes that night but instead they told all air defense units to NOT engage any potential targets without first receiving permission from the headquarters. Why ? Because the HQ has access to much more advanced radar systems and a broader view of everything going on. The air defense unit near the airport had parked facing the wrong direction. So they thought that the plane leaving the airport was actually a US plane / missile heading towards them. They panicked and without following orders, without receiving confirmation they shot down the plane. They have already been convicted and imprisoned and the families have been offered compensation for what happened.
Most of the families have accepted the compensation but a few holdouts are suing the Iranian government trying to say that Iran shot down the plane on purpose, completely disregarding the report. That makes ZERO sense. You know why ? Iran has the largest missile program in the region and a very potent air defense network. Everyday dozens if not hundreds of passenger planes fly over Iranian airspace. If the Iranian government wanted to they could easily shoot down countless passenger planes. However they don't. So either these Iranian family members are holding a grudge or they just want money (which is sad to say) or both.