My thoughts on why Murmur instead of someone with low-level telepathy or bestial communications skills... well, maybe they wanted to play with the old plotline of Kara using her powers indiscriminately and selfishly. The difference, though, is that Kara had a good heart... and we can't really say the same for sure about Murmur, since she was so obviously under Department H control. She tried more than once to use her powers to kill (albeit, she was being controlled by Mesmero for one of those incidents). Murmur was a wildcard that Department H enjoyed having, because she could (if so ordered) make any member of that team do anything she wanted.