How to know if a dentist did wrong purposely?

[toothache10002]

I had a cavity fixed 6 months ago. I thought it was all good. But now I'm told it didn't go well and I need a root canal. The X-ray shows the gum area around the tooth is all dark. How could this happen?

 

[Dr M]

Good day

Do you perhaps have any x -rays before or after the procedure? Difficult to give any advice based on the limited info