Regardless of your reasoning, and even if you initially accepted the job, you really don't owe them anything. I find it very strange that they would reply to you, chastising or grilling you that way.
This kind of thing happens to employers all the time! It's really no big deal at all! Usually, when a person is job hunting, they go to as many interviews as they possibly can, and so they often end up receiving more than one job offer in this process, as it's a good idea to always accept the first offer you receive while you continue to search for the ideal job, in case it's the only offer you get! It's very common for somebody to initially accept a job, then later receive another/better offer, and ultimately choose the latter.
I can't tell you how many times I've personally sent a prospective employee an offer, only to find they would not be accepting for one reason or another, or how many times a prospect has accepted the job, only to call a couple days later saying they could not, for one reason or another, accept after all (usually another offer that they preferred for whatever reason). It happens all the time!
I would expect any reply to the email you sent to only say "Aww, oh well, thanks for letting us know! Good luck in your new venture!" or to inquire whether there was anything (£££) they could offer that would entice you to accept their offer over the other one.
That said, I think, if it were me, I would reply to the email, but only so that I don't burn any bridges in the event that I do end up wanting to work there in the future.
Personally, I would probably just lie and tell them that I received a better offer in the meantime, and say that I told them as soon as I found out.
But there is nothing wrong with telling them that the reason you declined was because when you really thought about it, you realised that the job wasn't exactly what you thought it was. In fact, they may appreciate that feedback, as they probably want to update the posting to be more accurate or re-think their interview strategy or something.
Hmm... actually... I wonder if this has happened to them before, and perhaps that is why they replied with the nastygram? Because people keep declining their offers? Hmm...