Ben had to keep Locke alive so that he could get Eloise Hawking's
name. Once he got this, he killed Locke viciously, but he probably
knew that Locke would come back to life once he reached the island.
This is why he preserves the body so carefully. Therefore, Ben isn't
necessarily bad - he may just be doing what needs to be done in order
to get everyone back to the island. I think once he'd convinced Locke
NOT to kill himself, he would have been unable to then re-convince him
TO kill himself without seeming suspicious. He had to do it himself,
and he had to do it as quickly as possible because his time is limited
in getting everyone back to the island.
But, if I'm right, we dont know why Ben needed Eloise Hawking's name.
Clearly, the two know each other. Could it be that Eloise is some
kind of eternal conduit to the island and Ben keeps showing up on her
doorstep despite the fact that her identity is always changing? Maybe
Ben needs Jack's father's (Jacob?) cooperation to get to the Lamp Post
and he gets it through Locke this time since he is no longer in
Jacob's favor?