Ed turned Ben into one of the undead so he could have another of his species for company.
Think about it. If Ben and Ed really did have some sort of friendly relationship, they would have been unable to communicate due to the sort of "language barrier" between humans and zombies. Ed got a free meal and made his only real friend into the second of his kind. Hans is crying in the credits because he actually believed Ben and Ed were friends, and wasn’t anticipating what Ed would do.
If you look around at the advertising, it specifically refers to Ben as Ed’s best friend. On the surface it seems like this would be to throw the player off from the ending. But really, what reason would Hans have to advertise it as such? The event would be functionally the same, and the premise would, admittedly, be a bit of a risk to take, and very few people would believe it as a cover. It’s also not like making an animal perform for its meal is a rare thing either. Rundead could’ve easily been marketed as such. The only reason that makes sense for marketing it as Ed running to save his best friend is that Hans legitimately believed that Ed was Ben’s friend, and was absolutely horrified to watch him devour Ben instead. Ben getting out to the surface is the beginning of the zombie apocalypse.
We never hear about any other undead out there. We are also specifically told that Ed is the first undead, which seemingly doesn't relate to the plot at all. However if we look at 'the first undead' as at 'the one who began the outbreak' and connect it with the fact that the only person that Ed has turned is Ben, it is not too much of a stretch to asume the end of Bencalypse mode (even the name confirms!) marks the beginning of newly turned hungry Ben's rampage during which he turns more and more people into the undead. Really, are we sure there is no Word of God on that?