It is the Greys, working closely with the Dracs (Alpha-Draconians) who are the "bad aliens" in most of the Earthling news reports and "UFOology." The Zeta Reticulans are not here to harm Earthlings, harvest them, control, eat them, or any other such purpose. There is a long (and long) history, some of it rather dramatic, for the Zeta Reteiculans and the Zeta Reticulus star system, including the famous Lyan / Orion wars and the Alpha-Draconians. Large portions of it read like a (very long) Earthling science-fiction action/adventure story ("space opera" being the old term, but rather applicable here).
The Zetans are neutral (mostl) concerning Earth and Earthlings. Part of their purpose is the preserve the Eartlings largely untouched, uninfluenced, for purposes of study. The simple fact is that Earthlings -- Earth Descended ("ED") Huamns -- remind them an awful lot of themselves. Not physically, of course, certainly not technologically, but emotionally. They hope that in watching and learning about the ED Humans they can perhaps understand themselves just a little bit better.
So, in short, the Zetans have two purposes here. First, to study the Earthlings, observe and learn. Secondly, to preserve the Earth in its "natural" state, so that Humans may continue unfolding on their own, for good or bad, survival or death.
This puts the Zetans some what at odds with all the other Council member races -- especially the Greys and Dracs, but even with the Pleiadians, though the Pleiadians and Zetans certain get along and are quite civil to each other (these days, at least -- the Lyran wars were (after all) a very long time ago. In wanting to preserve Earth as it is, or at least to hinder outside influences upon it, they are more likely to support some of the rogue mission groups, than they are the major groups. That is, the Gangsters and Joy-Riders are more likely to receive help from the Zetans than are the Harvesters, Manipulators or Light Workers (see the Mission category summary articles).
This puts the Zetans into a difficult position, actually. Ideally they want to study the Earth totally natural and unaffected. In reality, because there are already so many outside influences active here they end up having to take a very active hand in attempting to keep things as they are, restore things to how they "should be" (using a best-guess approach, of course), and so forth. Naturally, this causes a fair amount of criticism and the expectation that any results gleaned from a study of the Earthlings can not possibly be pure science, any longer.
The Zetans, howver, remind their detractors that, first of all, ED Huamsn are not even natice to the planet anyway, secondly that outside influences have been active on the planet for thousands of years, and thirdly that the (so-called) "Yuga Cycle" the Earth regularly goes through is most definitely an outside influence already. So why not study anyway? Yes, it's a complex system to study, but so is every thing worth the effort of understanding.