Again, this is very simplistic reasoning (based on the very faulty premise that IS only exists to cause terror) that doesn't actually support your claims that IS would a) use a nuclear weapon should they come to possess one and b) that their current atrocities are worse than dropping a nuclear bomb on a populated region (
source).
Everything they have done so far suggests that they aren't just a bunch of cave-dwelling sons of construction magnates -- they know how to fight a war and formulate strategies. A single nuclear weapon, should they come to possess one, is of far greater use as a threat than if it were actually used. I doubt that it has escaped the notice of IS leaders that their enemies -- that is to say the West and Russia -- have many more nuclear weapons than they could ever hope to possess. Enough to render their puny caliphate uninhabitable.
Given that there is no great evidence that they definitely would or would not use such a weapon, this kind of speculation is all we have. What makes my speculation more valid than yours? It's logically extrapolated from what we already know about them and doesn't try to masquerade as absolute fact. You've made two assertions ((a) and b) as above) that aren't supported by your reasoning. There's no reason to presume that their butchering of civilians is evidence for what they would do if they got their hands on a nuclear weapon.
You seem to think that their treatment of civilians using conventional weapons (yes, I'm aware that it's not restricted to beheadings...) is somehow directly comparable to what they would do with a nuclear weapon. It isn't. Cutting off people's heads, raping women and children, throwing homosexuals from the tops of tall buildings -- these are all things that, while rightfully considered abhorrent by the West, they're more or less sure won't be responded to in a similar fashion. They are just not seen as serious enough for the West to capture an IS fighter and cut his head off or rape his wife and kids. A nuclear weapon detonation, I would argue,
would be seen as serious enough to merit dropping a nuclear bomb (or firing a ballistic missile) on the Islamic State. The knee-jerk reaction from that, particularly among irrational people, would be too great to allow anything else.
Bottom line: the situation is a little more nuanced than you'd like to think. There's no basis in evidence for the claims you've made thus far, and you seem to think that they should be accepted by me as 'common sense' or as a way to fend off accusations of being delusional. Neither, unfortunately for you, is going to happen. I don't like emotional arguments.