While I think that the ripple effect of Flashpoint is wildly over exaggerated (Diggle is the only non-Flash character to even be affected by it), at least the Arrow-verse can make good use out of Barry’s selfish use of his powers.
Diggle just forgiving Barry isn’t good drama though, and Diggle taking major issue with what Barry did makes sense.
You know you’ve messed up when Diggle hits you with one of his patented lectures.
Diggle has always been flabberghasted by superpowers, and the idea that someone has the ability to change history on a whim probably won’t sit comfortably with him.