Yes and no.
They're not quite in the same league as Hitler, Stalin or Pol Pot. ISIS haven't reached the industrial levels of annihilating whole sections of the population, and never will. They're still a 'cottage industry' in the extermination stakes. The hostage taking is a bid to raise their profile, and 'Jihadi John' would run a mile if he thought the SAS or Navy Seals were on his back.
It's the Western, Central or Eastern Powers who have the yen for wholesale slaughter. We've had more practice, starting with the Crusades (still pretty local) 100 Years' War - a bit more widespread, 30 Years' War - going for bust right across Europe with the use of large armies and new-fangled cannon etc, WWI - really industrial scale, WWII - mega-global, no holds barred slogging matches like Stalingrad, El Alamein and Iwo Jima, Korean War threatened to go ballistic, Vietnam was almost ballistic.
Compared to that ISIS still has to find its feet, but they'd melt away before they lost too many. They're a mental threat more than physical: insidious, hard to pin down, elusive... In a word, 'foxy'.