Below can be considered spoilers for anyone who has not seen all of Season 3:
The Anime should have left Roberta intact too. I knew that ending didn't make sense to me. Now I know why.
I hope that the manga uses Roberta again.
Does anyone know why the anime decided to go against the manga's ending for Roberta? It just doesn't make any sense.
So, you want to know why? OK. I am a big fan of Roberta, and that ending (and overall the 5th episode) was a biggest pile of crap I've ever seen.
First of all, Roberta got strategical injuries for viewers to see she will not come back.
Second, whole OVA intentionally trying to make Roberta a villain, just to make other parties a heroes (Grey Fox).
And, the real answer to your question is just because they can , because there were no reason whatsoever to destroy her in the end.
Also, there is something about how her past catch up with her, how the bloodhound is dead, how losing limbs was a symbol of her decent into the madness, blah, blah ..., but those are a bunch of baloney (those are stuff people usually say when they have nothing smart to say).
There is a very large number of people who disagree with this ending, which, according to some sources affected OVA sales (I certainly won't buy something I don't like).
Like you, I would like to see her again because I really don't have character interesting enough for me to read the manga or watch the anime if they make a third season.