Given what a war like evil greedy people so many humans/countries have become ..... I think quarantining humanity would be the wisest thing to do rather than just eliminating their forthcoming problems ....:(
Your question presumes a lot of things we have no clue about: 1. FTL space travel. 2. Totally invisible to sky watchers. 3. Seamlessly infiltrate Earth societies and judge morality in small details. 4. Undetectable communications. 5. No devil to cloud their judgments.
If your alien race can do all those things, the human race is irrelevant by comparison.