Need a word for "Should not have happened' with a negative context

Solution 1:

Barring a more detailed example, I think that "regrettable" could be a good fit.

"The results from taking the wrong path were regrettable.

From the Merriam-Webster Online Dictionary:

"Regrettable": causing sadness or disappointment; deserving regret

Examples of REGRETTABLE:

"His decision to quit is regrettable."

"It was a regrettable mistake."

Solution 2:

I like preventable or unnecessary in your sentence. Both have very negative connotations without seeming rude. You could just go with avoidable too.

preventable: capable of being prevented; "conscious of preventable human suffering"