That is a significantly more fitting analogy, so good job with that. You obviously have culpability for threatening my life since that in itself is illegal. Do you have culpability for me blowing the head off of my neighbour? Morally at least, probably some. But like the street racing example the devil is in the details. Do you have an understanding of whether I'm armed or unarmed? And so on.
But there are issues with mapping your example onto Trump-Iran. Most are of the devil-in-the-details variety, but one is deal breaking: you're missing a step.
It's not one entity threatening another, it's one entity threatening another if they take a specific action against it. This is fundamentally different. Say you had given me a warning that if I harrassed your wife that you would kill me, after which I went and did it anyway, and then assumed that the attack was coming, and ultimately pulled the trigger on my neighbour by mistake.
No, under that scenario I would not hold you morally culpable in the slightest. It's my gross negligence, and the guilt is mine alone.
Though it is fun watching you guys invoking disproportionately conservative views on justice (felony murder doctrine) to try and pin the blame on Trump.
But we aren't talking about a justice system or a court verdict. We are talking about morality and politics. There is no over-arching state and legal system above all this, so it's nonsense to talk about 'felony murder doctrine'.
But if you want to make an analogy with crime - if someone kicks off a gang-war by killing a leading rival mobster, especially in a context where the police and legal system is weak or effectively non-existent, and subsequently innocent non-gang-members get caught in the resulting cross-fire and killed, then, yes, there is a collective responsibility by all the gangsters involved.
Even if one gang could be argued to be less vicious than the other, even if one gang could arugably be described rather as vigilantes, the affected public can ask 'what the heck did you do that for?' and consider them to have some share of culpability for the resulting carnage.