Benevolence is a matter of conduct not physical form.
Ultimately if the aliens are benevolent (in the true dictionary sense of the word) their actions and the manner in which they introduce themselves and deal with us will reflect that.
Also if they truly are benevolent they must by default have (or will seek as quickly as possible to obtain) at least a basic knowledge of humanity, its history and human society. This is because otherwise they wouldn't know how to explain or display their benevolence. After all if you want to perform an act of benevolence for some random person you've just met you need to get to know them and their background before you can do so.
So as other people have suggested robotic go-betweens or even just non alarming digital images/avatars would do for first contact and initial introductions while the aliens themselves remain in space. Once language and communications routines become established the aliens would be explaining their intentions and while doing this they would also explicitly state that the images/machines are not their true form. Being quite upfront about that point. Then?
They just tell the truth i.e. that they're worried humans will think they're horrible monsters. So they start with some basic facts about their biology and physiology and an explanation of life forms back home with images (stills first, later video) of their environment plus diagrams, then stills and finally video of their actual selves. They break the news gently from a distance and then wait for an offer/request for a meeting from us before face to face contact occurs.