They cause them unnecessary pain and suffering. If they didn't do this, their testing could be performed with greater ease and in a relatively shorter time, increasing the progress in their investigations. Still, if they weren't, destroying them systematically in front of humans would be pointless. Sounds a little bit unlikely. In order to answer this, we would have to take a closer look at these greys.
Many people describe the greys as being emotionless.
Also, native believes state that the greys are not even extra-terrestial in origin, but that we are dealing with demons. If this is so, then this would indicate that we are dealing with beings from a lower level of existence. Apparently they are following their own agenda, and not concerning themselves with ours. If you are one of those people who's been abducted at any time, think about whether you really want these beings to control your life and whether you really want these beings to supress your free will, to manipulate you and whether you really want to be part of their egocentric game that they are playing with humanity. The general public appears to have a positive image of these beings even though they hurt, mutilate, kidnap, torture and abuse human-beings on a regular basis.