He was completely negligent in not keep them contained on his property thus allowing them to do what they are well known to do, kill and maim.
If you choose to own dogs that are of a breed known to kill and maim at a rate much higher than any other breed and then negligently allow them to escape confinement and attack innocent people then yes, you should be held accountable exactly as if you had done their deeds with your own hands.
It is clearly his fault in every way. It is his fault, and on one else's, for owning them, not securing them, and therefore allowing them to kill this lady. She would still be alive if not for his bad decisions and negligence.
Every single step in this is exactly his fault.
No. Manslaughter, yes. Criminal negligence yes. Genuine murder, no.
Dogs are not the same as guns. If one can be negligent with guns, one sure as heck can be with a being that moves on its own and has a mind of its own.