OK, this is a plague among us. I don't care if it makes me sound old, but the current generation of 20somethings are very much concerning me.
Here is the deal. When an employer hires you, they invest time, money and energy in getting things ready for you to begin your employment with them. To email them the day that you are meant to start your employment and turn down the job is inappropriate, disrespectful and wrong.
First, when you accept a job unless you have prepared the potential employer that you will be continuing your search for work so they can make an informed decision, you now have a job. Stop looking. You have committed and should honor that commitment. It is dishonest to accept employment and then bail.
Second, under no circumstances is emailing your "resignation" the day you should be starting appropriate in any circumstance. On those rare occasions where another opportunity is of significantly higher value that you have determined it is worth breaking your commitment (as the man said to the woman, we have determined what you are, now we are just negotiating) you should give as much advance notice as possible. If that is not possible, then you owe it to those who were willing to give you an opportunity to explain yourself in person.
I have historically thought as an employer, better to lose them now than two weeks or months into it. I still think that. But the level of disrespect and lack of honor is becoming rampant. Have some pride in who you are, and be willing to stand up to your problems. If you are going to make an unpleasant decision, fine, but have the guts to own it and mitigate its effects on others to the amount you are able.