It was a bold move by Trump. I'm not entirely up to speed about how much of a bastard this Iranian guy was. He seems to be responsible for a lot of deaths.
There's a lot of bluster in Iran about revenge, war!, etc. I doubt they'll do much - just look at their standing army, air force and navy. If the Iranians got stupid it'd be Shock and Awe Mk. II.
Just as an aside, notice that there's been little real real outcry from Muslim communities in the West.
There has been little flag burning in places such as Pakistan.
Why? Well, the vast majority of Western Muslims and Pakistani Muslims are Sunni (globally Muslims are about 75 - 85% Sunni, the rest being Shia, Sufi, Ahmadiya - with Shia being the largest minority).
And this Iranian guy was a Shia, as are most Iranians. He was also an extremist - he was the leader of the paramilitary Al-Quds force. These are Shia extremists who support Bashar al-Assad (who himself is a member of the Allowite sect, who are either a strand of Shia Islam or just align themselves with Shia Muslims) and the Iraqi government (which is Shia). Al-Quds and Hezbollah are both Shia extremist groups which have fought against the Sunni extremists in Syria and Iraq.
The question I'd like to ask is why is there an Iranian backed militia operating in Iraq ? - Iraq is approx. 65% Shia and 35% Sunni. Iran has a huge Shia majority - it is the fount of Shia Islam just as Saudi Arabia is the fount of Sunni Islam. The Iranian backed militia is known as Al-Quds and is Shia, as is the current Iraqi government. There is also a Lebanese militia active in Syria/Iraq, called Hezbollah ( حزب الله - 'the Party of God', 'the God Party'), which is also Shia. So, the short answer to your question is: because of sectarianism.