On a very simple level... it seems to be what people like to do these days. The US has funded rebel groups in Syria, that isn't with the intent of specifically going after Iran but rather they want to back group's opposing the regime (and for good reason!) - yes Iran is supportive of the regime.
That is somewhat different to actually backing groups and guiding them with the express purpose of attacking coalition forces - that is something Iran has done on and off for several years and well before the Arab spring/Syria etc..
That extends way beyond just supplying arms too - when the local Shia militia groups used to carry out attacks at Basra airport because the nimrod had been spotted on the tarmac by one of their dickers working on base then they were doing it on behalf of... you guessed it: Iran. Back when they couldn't penetrate the armour on British warriors (APC vehicles) guess which country stepped up to supply a different device... in fact guess which country funded most of them and was basically behind policy and whether they were going to carry on killing coalition troops or stop for a bit and only kill the new (at the time) ING (later absorbed into the Iraqi army) or police etc.. that got rather confusing as some police units were infested with Shia militiamen anyway so you'd get police cars attacked and the people behind it were in the same militia as some of their police colleagues. It wasn't just funding either - they directed this stuff, they had republican guard types in country.
Because they didn't - you're insisting on conflating different things - taking pot shots at the coalition in Iraq pre-dates anything in Syria... I'm well aware that the US and Iran have history but you might as well come up with some argument about the US was the aggressor because of X event in the 70s or 80s etc.. when it is BS. This was an attack in response to Iranian aggression, in Iraq, something that hasn't previously been responded to with direct action in this manner and has been going on for a couple of decades!
If you carry on making simplistic comparisons, expecting acknowledgement of "simple facts" to build simple arguments with no room for nuance then yes, you're right, there isn't much conversation to be had.
You’re first making the assumption that the US isn’t guiding its proxies to attack Iranian assets, which in itself is a leap of logic that probably won’t stand up.
Were also discussing perceived justification here. This is not whether you or I think it’s right or wrong, but the justification each side is using to attack the other. The US, whether intentionally or not, has caused the death of many Iranian military personnel. They have armed and given tactic approval for them to be used against Iran. There are obviously nuances, but the reality is just that. No different to Iran doing the same against the US.
If the US weren’t in Iraq, backing (in many cases) the opposing side, would Iran have attacked them? Probably not. In the same way if Iran wasn’t in Syria they probably wouldn’t have been attacked either.
As you point out, this proxy war has been going on for a while. The hardline elements in both the US and Iran seem to love it, as do US proxies and allies in the region (it gives them an excuse to act the way they do - Saudi and Israel being two).
Going back to the discussion about Iran hating the west. The politics is extremely complex in Iran. It has two very different sides, with the moderates trying to bring Iran back into the fold, and the hardliners trying to sabotage it. The moderates were the cause of the Nuclear deal, the hardliners are the cause of the attacks. The actions of Trump over the last couple of years have played directly into the Hardliners hands, and the assassination of an Iranian general is just the icing on the cake for them. They can now claim (somewhat legitimately) that Iran held out an olive leaf to the US, and now not only did the US (Trump) rip it up, he’s now “murdered” the messenger.
Iran needs to change some of its leadership, but it needs to do so on its own terms. These actions are just legitimizing and solidifying the support for the hardliners. Obama and his administration understood this, Trump doesn’t seem to. That or Netanyahu and Saudi got him to do their bidding - both were against the nuclear deal from the start, because it brought Iran in from the cold and damaged their political aims in the region.