You are right that most are just going to label this as just another terrorist gone without consideration to the scope of importance that not only he held but also that of Abu Mahi al-Muhandis (non deguerre for Jamal Jaafar al-Ibrahiami) also known as Hahd al-Shaabi, the fact they both were killed in the same air strikes, in my opinion, speaks volumes for considering that this strike couldn't have taken place without cooperation from players inside Iran, Iraq and also the United States.
Abu Mahi al-Muhandis is associated with the name al-Hashd al-Shabbi as he was the head of some forty umbrella groups that he brought together of mostly Shite's in the fight against ISIS, it was Iraqi state sponsored and it means or can be translated to mean: al-Hashd: mobilization, al-Shaabi: people or popular. It became to become also in short as Hashd during the time against the fight against ISIS. He was chosen as he was known as the figure head who organized these groups, including Kataib Hezbollah, which he played a part in founding, and was their financial figurehead. In the short term when they all had a common enemy, ISIS, it was all good. In the long term when they tried to incorporate some of these forces into the Iraqi formal security forces some factions, including Kataib Hezbollah still operated independently of the government. Abu Mahdi al Muhandis never fully developed Hashd into the ministerial command or became subordinate with the conventional forces of the Iraqi military, whereas these groups continued to viewed Abu Mahdi al Muhandis as their leader. During and therefore after he was an important figurehead in the direction and finances of these groups and as such he was rarely ever seen in public and it was extremely rare that he openly spoke in public or came out to make political statements, non communication is the key to keeping ones where about unknown.
How did he get on the Iraqi's bad side? After the fall of Saddam he was elected into the parliament until the US realized who he was and his involvement in the bombings of French and American embassies in Kuwait in 1983, where he was sentenced to death but he fled Kuwait before hand. He went on under the umbrella group of Kataib Hezbollah to hit at US targets in Iraq and was, in 2009, put on a terrorist list along with Kataib Hezbollah.
Qassem Soleimani, head of Iran's Islamic Revolutionary Guard, (Quds force) basically did the same thing with the clandestine groups he directed throughout Iraq, Syria, Lebanon and other area's of the middle east by organizing Shia branches of these groups in the fight against ISIS. Similar to the above it was all good while fighting ISIS but when that was no longer the initial focus, Soleimani and al-Muhandis had similar goals to disrupt the infrastructure of Iraq and other area's of the middle east. Soleimani became the chief of those operations and al-Muhandis became the deputy chief. Soleimani was considered the mastermind behind operational goals and al-Muhandis orchestrated the finances behind the operations. Soleimani became what some inside Iran would consider to powerful, as he often worked against the peace treaty protocols by running around the middle east unhinged feeling untouchable in regards with his relationship to the president of Iran. As such he was able to demand that certain others within the parliamentary make up of Iran to be left out of meetings. One reason one prime minister put in his resignation last year. Which basically would amount to what everyone is harping about on Trump about, he took the advise of his advisers without bringing it forth to congress. (Though because of the air strikes by Kataib Hezbollah and the storming of the US embassy Trumps moves fall within his authorization to act without notifying congress) Therefore it is completely plausible that there were some within Iran's government who felt Soleimani was completely out of control and needed to be dealt with. My speculating that it could be feasible that some within Iran, who knows...maybe as high up as the president, may have been involved in orchestrating this hit may be completely plausible. I say this in particular to the fact that something drew al-Muhandis out from underneath his rock, that something had to be reasonable strong to both men as you take into consideration that Soleimani was an expert in his calculations, one who wouldn't miscalculate the damage that could be done to the umbrella organizations if both of them were lost/killed at the same time.
My assessment is, (opinion), that these two men became a liability to all three governments, Iran, Iraq and the United States. I think they were all grappling with how to deal with their growing "umbrella" terrorist organizations, though those organizations had a common enemy within themselves when it came to the United States they could also be hemmed out and specifically focused towards acts against those within the government(s) of Iraq and Iran dependent upon policies and/or politicians who may act in a manner to which they have disagreement. They themselves, through their incredible ability to control, finance multiple organizations of terror throughout the middle east became the common enemy. If true what I think we will see during the coming weeks isn't all out war with Iran but strikes perpetrated by the effected branches of those organizations once controlled by these men once they figure out how to reorganize. They each will be independently dealt with as they happen as there is no reason to believe that they will not. Short of the Iranian president televising an announcement of open war this is how we should be calculating the meaning against any terrorist strikes, as that will be what they are, terrorist strikes.