It's not a matter of a "right to life". It's a matter of who is making choices in that life. Even if the Monster is sapient but ignorant, that doesn't change the fact that the Monster is making decisions without the influence of Frankenstein (he's just making those decisions from a place of ignorance, which is further proof Frankenstein did NOT influence the Monster towards killing people). If the Monster were even truly physically and mentally identical to, say, an average 6 year old child, if that small child committed a murder you couldn't blame the father of that child; epecially if it was a father who abandoned them at 5 years old. Sure, what the father did in abandoning the child is despicable, but in the interim period the child showed personal autonomy and made its own decisions. It was acting outside the sphere of the father's influence or control. The father can be held culpable of abandonment, negligence of the child's own safety, etc., but he is not culpable for the actual acts of murder - those were completely the idea of the child (here "Monster").

The deny the Monster this autonomy, then you are denying the Monster's humanity entirely. So, is the Monster truly an inhuman monster incapable of the free will all humans share? Or is he a man, but a man that chose to kill?

(Just as an aside, I want to make it clear that this is just how I would argue this point to defend Frankenstein. I don't actually believe that Frankenstein is worth defending, but that's the assignment at hand.)