If there are any aliens who can visit Earth, they may be "good," "bad," or even "neutral." It is impossible to know what their motives for making contact with Earth are until it actually happens.
I don't think aliens can be described as "good" or "bad" overall. Most life on other planets will probably never come in contact with Earthlings, so they aren't good or bad by Earth standards. Most of them are probably just living their lives on their own planets, just like us. Many inhabited planets may not even have "intelligent" life that we would recognize as being similar to ourselves, and may more closely resemble animals, plants, or bacteria. As much as humans like to place different things into discrete categories, we can't really categorize aliens as being "good" or "bad."