Why Andi’s email might be discovered if Miles were to commit murder?

Question

Andi, Helen’s sister, sent an email to several people, including Birdie Jay, Duke Cody, Claire Debella and Lionel Toussaint, threatening to use a red envelope she found to burn down an empire. The next day, she was found dead in the garage with sleeping pills in her system. Helen went to Detective Blac with a box to find out who was responsible for her sister’s murder. After meeting him, she tried to relate Andi’s murder to Miles:

Helen: What about Miles? What if he just did it?

Blac: Well, we can't rule it out, but Miles Bron is not an idiot. To risk committing murder after a very public court case, with the possibility of that email of Andi's coming to light, would be an exceedingly stupid thing to do.

Why Andi’s email might be discovered if Miles were to commit murder?

Answer 1

It's not that killing Andi makes it more likely the email is discovered, it's just that it is very likely to be given to the the Police by someone when investigating her murder.

The existence of the email and the threat she makes in it, is motive for Miles to kill Andi.

Answer 2

The point of the comment is to future reference that Miles Bron actually is an idiot.

Blanc is, initially, assuming Miles' intelligence based in what he knows but later on he says...

BLANC

His dock doesn't float, his wonder fuel is a disaster, his grasp of disruption theory is remedial at best, he didn't design the puzzle boxes, he didn't write the mystery - and voila, it all adds up, the key to this entire case, and it was staring me right in the face. Like everyone in the world I assumed that Miles Bron was a complicated genius. But why? Look into the clear center of this glass onion:

beat( )

Miles Bron is an idiot

Nevertheless, once Helen is murdered a motive must be found and on investigation the email would surely be mentioned by someone and Miles would come under suspicion.