The Americans have confirmed that IS has used missiles with mustard gas warheads in fighting near Mosul. For the unprotected this can maim or kill, and it could be made in your kitchen.
Sarin, a much more deadly chemical agent, is also not that difficult to make for a chemistry technician, and the raw materials are also readily available.
It is the ability to deliver these weapons that worries the Americans. Russia Today (RT) is reporting that there have been various chemical weapons attacks unreported in the media from both sides. RT also says that VX, the most toxic substance, might also have been made by IS. If IS can get that in a warhead and deliver it all bets are off. Everybody will join in against them.
Put it in perspective. If VX was released in a crowded city, Birmingham, Manchester, Leeds, the London Underground, Moscow, New York, the deaths would be in the thousands.
Should IS take the chemical route the end game might be nearer than it thinks, and it will not be nice. Reap what you sow, and all that.