Social media isn't about, and has never been about, freedom of expression, as much as sometimes we seem to think it is. It's about saying whatever you need to to get scale, and then suppressing dissident communities that might cost you ad revenue or you personally find distasteful once you have the power and authority to do so. It's a cultural appropriation of a hacker ethos for the sake of a business purpose.
As for this action, very few things enrage me as much as Wahhabism and Islamic extremism; in fact, sometimes I struggle with the cognitive desire to write off Islam and the Muslim world as a whole because of ISIL and its followers.
However, I don't believe that this idea makes much sense. You cannot kill a meme by driving it off your platform; religious zealots have traditionally faced much worse persecution than being banned from a service. They will spring up again in places harder to detect, with tools that strengthen their communities. Their enclaves will no longer coexist alongside peaceful messages; they will be harbored entirely within their own safe spaces, echo chambers that strengthen their hatred in their isolation. Instead of a potential ISIL sympathizer seeing ISIL messages alongside trending topics against extremism and terrorism, they will spend time on extremist forums in which there is no dissenting opinion. They will not see pictures of people lamenting the death, they will have communities where this death is entirely praised. Instead of seeing football scores or other young people facing their same problems that may have differing religious opinions, they will find more people willing to indoctrinate them when exploring whether or not they find meaning in Islamic extremism.
Twitter may think they are taking the oxygen out of the atmosphere by banning them, but in reality they are simply shifting the fire to someplace where it can burn hotter.