Think geopolitically. Everything ISIS does is to a geopolitical end, and this.......murder, is ultimately but a part of their wider geopolitical ambitions. The same can be said for any actors on the international stage, not just ISIS: however, given their recent prominence it is naive to assume that they do not play by the rules by which the world works.
By murdering Muadh Al-Kasabeh as horrifically as they could (and it is genuinely horrific: the whole video's up on Fox News, though I'd strongly recommend staying as far away from it as possible), they probably hoped to achieve two things: firstly, to frighten the Gulf monarchies into reducing their air operations against ISIS, and secondly, to force the USAF towards carrying out less risky air attacks from a higher flight envelope. The murder of Al-Kasabeh has generated anger in Jordan, and a renewed response from them, but notably, other Arab monarchies, chief among them the United Arab Emirates, were shaken enough to suspend all air operations in Iraq and Syria until further notice. The killing of a UAE or Qatari pilot by ISIS in a similar fashion would not inspire the same response that it did in Jordan: both these countries' monarchies do not possess the legitimacy that Jordan's sheikhs do for a variety of reasons, and the idea of both countries as independent nation-states in their own right is shaky at best, unlike in Jordan. Hence, Jordan's nationalistic fervor would likely not be replicated in these countries if their own pilots were killed by ISIS: instead, they'd see a renewed wave of Islamism gripping their mainly immigrant societies, and the thought of that is likely scary enough to the monarchs of these countries (who aren't exactly paragons of Islamic virtue) to account for their suspension of attack operations against ISIS.
So, ISIS has already probably achieved something tangible by killing Al-Kasabeh in this fashion. Their other possible objective, from a geopolitical point of view, is, as mentioned before, to force the USAF to conduct its operations from higher altitudes further above the maximum range of ISIS's surface-to-air missiles: by doing this, the USAF will have to sacrifice accuracy for pilot safety, and ISIS probably hope that this will a) reduce their casualties, and b) increase civilian casualties, which will only radicalise the population further and turn them towards the IS. There is also a possibility that the US will have to station search-and-rescue units close to Iraq and Syria to prepare for the possibility of another pilot being downed, and this gives ISIS an opportunity to engage US troops on the ground and possibly inflict further casualties and/or capture US personnel.
There is method in their brutal, horrific sadism. I only hope the decision makers in the US now see this and figure out methods to destroy them that take into account their rationality: I get the feeling that the US has too often turned rational enemies into monsters, and acted accordingly. The problem there being, monsters aren't rational: they can only be killed, not defeated. But in the real world, just mindlessly killing 'monsters' doesn't end the problems that spawned those monsters.
ISIS are desperate: their killing of Kasabeh indicates that the air campaign is working, and working well. That needs to be firmly in the minds of the decision makers in this fight.
And as for Captain Al-Kasabeh, may he find peace. If the people in charge really wanted to avenge him, they'd go after the Gulf states whose financing and support created ISIS, and ensure that they fully commit to killing the beast they themselves reared: unfortunately, geopolitics rarely works that way.
(I have a feeling there might be some)