This opinion piece will contain speculation or "conspiracy theory," and so you can move on if you don't like that sort of thing...
I'm not claiming that Iran fired any missiles at the Ukrainian passenger plane; that's the New York Times and Bellingcat. But, assuming their video shows unaltered footage of Iranian anti-aircraft SAMs shooting down Ukraine Flight 752, then we must ask the obvious question: WHY?
The very first possible reason screams out, and that is that Iran must have thought it was an attacking warplane over Tehran, and with only moments to decide, they shot quickly. But, why would the Iranian military think that a civilian plane was an attacking warplane? That is the big question in this possible scenario, and it represents a major threat to civil aviation worldwide.
Other civilian airliners around the Middle East have been put at risk recently in the following way. In December, 2018, Russia accused Israel of using civilian airliners as cover for their attacking warplanes:
"The provocative actions of the Israeli air force ... directly threatened two airliners," Russian defence ministry spokesman Igor Konashenkov said in a statement.
In another incident, Israeli planes attacked Syrian ground targets in a way that the Russians said caused Syria's air defense missiles to strike a Russian surveillance plane that was in the same area. This air defense response killed fifteen Russian airmen.
"The [Russian] spokesman accused Israeli pilots of "using the Russian airplane as a cover", putting it "in the line of fire coming from Syrian air defence systems".
These two precedents establish that military strategists are always looking for ways to game the enemy into using his anti-aircraft missiles against him, so to speak. Getting the enemy to shoot down the wrong plane is a standard military ploy, and perhaps a war crime.
Civilian planes are identified by their radio signatures. They continually broadcast data identifying them as friendly, or, in the case of military aircraft, they can turn off transponder signals and be unknowngiving away as little information to the enemy as possible.
Radio jamming has been around since the second World War. It is not impossible that the Ukraine jet was targeted for some sort of radio jamming in order to confuse the Iranians into thinking it was an attacking enemy warplane and quickly shooting it down. The incident would clearly be blamed on Iran alone, as the "conspiracy theory" is nearly impossible to prove, and the Iranians aren't going to admit to shooting missiles at all, just as they have denied the use of missiles ever since the incident occurred.
This scenario seems plausible enough. While pundits allude to Iran making a "mistake," this absolves any conspirators who may have engaged in deliberate radio jamming in order to bait the Iranians into shooting down the wrong plane. The propaganda value of casting the Iranians as monsters, and also angering Ukrainians, Canadians, and other nations which had civilians on board, are clear motives to inspire such a crime.
Lastly, it makes no sense that Iran would warn the US about its missile launches hours in advance in order to avoid casualties, while deliberately shooting down a plane load of innocent civilians on the same day.