Why do people romanticize the Vikings but despise Islamic terrorists and ISIS? Aren't they the same?

The Vikings were a lot more than just raiders and rapers. They were farmers, traders, they had a rich culture of poetry and art, et cetera.

However, you are correct that the Vikings did, however, do things that ISIS is accused of. Vikings certainly did raid villages, slaughter peasants, rape women, et cetera.

So why do the Vikings get a free pass whilst ISIS does not? I think the simple reason is that the height of the Viking age was over one thousand years ago. If there was some terrorist organization operating in the North Sea that was launching raids into England or Northern France or what have you in the present day, they would almost certainly be condemned by almost everyone.

It is very common in society - both in our era and in past eras - to romanticize past cultures. Given that warfare, violence, and martial strength has long been something romanticized by many cultures, famous groups of "warriors" frequently make their way into myth and legend in an not entirely negative manner. The Vikings aren't alone in this regard. Take the Spartans, for example. The ancient Sparta that people love to lionize, the Sparta of fearsome men and strong women, was only made possible by the fact that the Spartans ruled over a huge amount of people known as "helots." These helots were basically serfs, or slaves, if you want to be less charitable to the Spartans. They were mistreated and misruled, disenfranchised and abused, and it is their toil that allowed Spartan citizens to devote their time to martial training. Yet Sparta is given a free pass for this by many people, whilst modern states that sustain their armies in part by exploiting a poor underclass, like North Korea, are villified. And this is because the Spartans don't exist in the modern era, they existed in classical antiquity. Hell, many of the Spartans enemies weren't exactly saints either - the Athenians, for all their philosophy and intellect, had no problem killing the men and enslaving the women and children of Melos, a city that refused to join their side in the Peloponnesian War.

Vikings, Spartans, samurai, knights (to a lesser extent, perhaps due to Renaissance "dark ages" ideas people still often talk about how knights were bullies who oppressed serfs and peasants), nomadic tribes like the Mongols, et cetera, are all celebrated in the modern era by different cultures. These warriors might have committed acts of brutality in their time, but they succeeded in leaving a mark in the cultural memory of the societies they existed in, and someone in 2014 who likes to talk about Vikings or samurai probably isn't actually advocating bloodshed.

Romanticization should not get in the way of studying history, of course, but it isn't an inherently evil thing. It can definitely be used for evil - like how the Japanese Military in 1920s-40s used warped versions of the samurai code of "bushido" as a form of propaganda and indoctrination - but it is often just innocent fascination.