That would be only logical thing for them to do if we assume that they are more advanced in technology and ofc, more advanced in every spiritual way. If they would only see few chapters of our planet history... In almost every decade of our long history there was some terrible thing done by humans to other, mostly weaker people. Not to mention what we do to animals, so many kinds of wonderful living beings are lost forever.
Then, for example, there were many atrocities done in time of colonization of new worlds. Some scarves from that times are visible even today. Im sure that even a cave man, after he found about fire he used it to burn someone, just from curiosity and today we have nuclear bombs - such a wonderful achievement ( even we gave them a pacifist names, like ' the keepers of peace ' )
It doesnt matter if, when used, such instrument for peacekeeping would destroy hundreds of thousands lives and who knows how much land would be unusable for years.
As u said, they would prevent **** from infect other parts of universe and it doesnt matter how many good people there is in the world when we can see with every new day that evil has indeed won or on a road to take over this planet.