How does one mistake a 737 for a tomahawk missile?

Mistakes can happen.  Ask this guy how he thought an Airbus A300 was an F14.

https://en.wikipedia.org/wiki/William_C._Rogers_II...

To the outsider, it certainly does look unlikely in the extreme for this to be an accident, but you have no idea how little training and supervision the staff has. 

While I have less specific knowledge of Iran's military, I have personally witnessed (mostly in Russia and Kazakhstan, among other countries ) illiterates, new "trainees" , religious or political fanatics (with no background, training, or supervision), operate complex military and aviation hardware/software. 

This included 17 year olds working 36 hours straight, people literally too drunk to operate  a telephone, people with such poor vision they could not drive a car, people in charge who were reading manuals for the first time, and more.  

So, while it seems improbable, I'd say it is at least possible. 

In most western/modern countries, it is assumed (and is usually the case) those operating such systems are extremely well trained, well supervised, and have gone over  existing protocols for hundreds of possible scenarios, possible malfunctions, possible mistakes. In many other countries, the job is done by someone's nephew who really needed a job of some sort and had already been fired from their job as a cook, cable installer, or mail boy. 

It was bound to happen, notice there were no Americans on the plane.. cold calculated murder of the the people on that aircraft.

Apparently Russian anti-aircraft systems have a problem(s) figuring out civie aircraft from military aircraft??  Someone should have told them that aircraft leaving Iranian airspace are likely friendly aircraft.  And not attacking aircraft.  

Russian anti-aircraft systems are famous for shooting down friendly civie airplanes.  Ask the Ukrainians.        

Modern weapons 'lock onto' something moving in the area.   So it's quite possible with rockets flying around out there, for this tragedy to happen.  More to the point was why was this passenger flight allowed to leave with that going on?

Trigger happy crew, scared stiff of an incoming missile hitting them...

Not easily, but the Iranians lied about even blowing the plane up until they were looking ridiculous. Now they admit it, but blame trump for making them. 

Such petulant  babies

Well you seem an unbiased and perfectly reasonable, mentally stable and balanced chap.  We shall collectively bow to your wisdom.