which makes it an illegal act in international law.
And here is where it gets interesting ...
What is the definition of "Mercenary"? because pre-ISIL and ISIS Iraq had private military company operations in the area supporting US military and civilian activities (included but not limited to the whole Blackwater scandal). Non US military elements were operating to support foreign government operations, under a licensed permission granted by the invading country (Because the US was invading Iraq, not liberating it from anything).
Under Iraqi law, those invading forces were technically mercenaries so there is an understanding of this word that need to be accounted for, if there is a warrant or bounty on suspected terrorist and they pass or assists in such activities (and they are not members of the military) they effectively become bounty hunters (which is not a crime by international law due to them being support of local law enforcement).
I guess I can grab all my guns now, and head to Mexico to kill the gangs down there, since according to Trump, and about everyone else in the GOP, we're having a "war on drugs." So under your understanding of the situation, any illegal formed group can be sought after and slaughtered... scot-free
So under your understanding this can't be done, however the we had a similar Mexico gang getting out of control situation, it didn't stop until US and local law enforcement joint forces to take down cartel leaders, and I believe you need to read more.
https://en.wikipedia.org/wiki/Pablo_Escobar#Search_Bloc_and_Los_Pepes
What you don't seem to understand is that there is applicable local law that does not follow international law, everyone does it, even the US.
I'm pretty sure ISIL sympathisers will justify their actions in exactly the same way - that they are reinforcing anti-criminal operations by supporting the entity that they believe has the moral authority in the area.
What you're forgetting is that while we consider ISIL to be terrorists, ISIL and their sympathisers don't think of themselves as terrorists. They believe that they are justified just as vehemently as we believe that they are not. There is an expression that the law is an ass - that it is stubborn and inflexible. It must be impartial and it must be applied equally and evenly. As soon as you start applying the law differently because it is convenient or popular to do so, you undermine the entire legal system, and you undermine society as a whole. We become everything that the terrorists say we are - hypocrites who think that it is okay for one person to go off and fight for what they believe in, but not okay for another person to do it.
What you are forgetting is that ISIL
are terrorists; there is Human trafficking, drug dealing, oil smuggling, genocide of minority groups. Just as the Nazi Leaders and commanders ISIS pretty much got sentenced to death by Iraqi, Syrian and International law when they starting the activities, so local law enforcements will not bother if there is assistance while fighting the problem.
I understand is a chaotic situation, however applicable law is subjective by who is doing the fighting and it's side, this environment is created by the lack a serious international ground force supporting operations, in other words, this will keep happening unless there is an international force on the ground.
Have you ever seen Tarantino's Inglorious Basterds? There's a scene where the Basterds question a German soldier about troop movements. When he refuses, they execute him by beating him to death with a baseball bat. The German soldier is presented as being loyal, polite and honourable; all qualities that the Allies valued. He just happens to be German and therefore fighting for the Axis. Meanwhile, the Basterds are established as the protagonists of the film and represent the Allied war effort. They're cast as the good guys, but they do horrendous things. The underlying message of the film is that the Allies could have done more to fight the Nazis - but it wouls have meant ignoring the law and putting aside morality, and by doing so, they would have been no better than the Nazis. I believe that the same applies here: we cannot ignore our own laws and dispense with inconvenient morality to defeat ISIL because then we lower ourselves to their level.
So your idea is that killing ISIS is bad because they ... are people, and is morally wrong to kill them.
What you forget is that each ISIS fighter is defending it's 2-3 wifes given by ISIS command, is the same person operating to control areas and population that will later be taxed and it's population subjected to ISIS rule (so they become slaves, or if they failed the aforementioned Islamic moral code they will end up dead).
So no, I don't have a problem with ISIS fighters getting killed by mercenaries, those are two ends of the moral spectrum in a place where morality is just a word rather than a set of values, there is no such thing as a "A lovely ISIS fighter" who thinks is defending its bible and the sex slave wifes given to him by Clerics leading this nonsense.
ISIS religion is not the same Islam faith other country practices (even Islamic countries say this), so can we people stop assuming that fighters are indoctrinated when in much of the cases they are not, and they are just looking resources to keep their nonsense going (and the truly god awful perks that comes with it).