How can we go from that "it" to what it means to be a good Muslim? (an "ought").
Because the book is very clear that the example of the prophet mohammed must be followed. The closer you follow it, the more Muslim you are. I notice you still haven't even attempted to answer "why aren't ISIS Muslim" like so many of your bleeding heart liberal politicians.
Because. You. Can't.
Well, you can, but the answer goes against everything you want to believe. So you pretend the question doesn't exist or try to change it into some English grammar test. Really, who gives a flying fuck about "it", "is" and "ought". When they are coming to butcher your family, you should lecture them on how they "ought" not to be doing "it". Good luck with that.
Why are they meant to imitate it?
Because it says so.
Why can't they pick and choose like modern Christians/jews do, or organize their beliefs around a church hierarchy/tradition like the Catholics, or whatever?
Because their book isn't as vague.
Luke talks about a burning hell, IMO one of the morally corrosive ideas both found in the Quran and the Bible. Does that mean a person who does not believe in a burning hell is not a "good Christian"? I don't think so, I certainly don't think there are any logical reasons for it.
Believing in a burning hell doesn't hurt anyone else, so very very different. It's also open to interpretation whether it is meant figuratively or literally but it doesn't matter, because it's just a thought about something that happens in some fairy land and, importantly, the only impact it has on our world is to try to prevent people from committing serious crimes and sins.
Following the example of the prophet with a clear documented life story of what is "great" (killing people) and what isn't (being nice) is much more direct. And it's effect is to encourage people to commit serious crimes.
Can you see the difference yet? Of course not, there are none so blind as those who apologize for Islam.
So let's give you another go: are ISIS Muslim. Yes or No. Are they more or less Muslim than "moderate" Muslims. yes or no.
Explain your reasoning of you can come up with any.