Maybe he did just decide to. Maybe not. But you or I, as normal humans could decide to. You could go kill someone now, you don't need a reason to justify it to yourself. We can act illogically.
This contradicts my point of view as you can understand. I say everything has a reason, so he must have a reason for murders. Every serial killer has a pattern and a goal. They want to give a message to the society. They are killing people to punish some other people. No, we can not go kill someone without any reason. If you just stand up, go out and kill someone now, the reason for you to do that will be to prove that you can kill without any reason. But this time even you will have something to prove and this killing you just did will have a purpose.
We act illogically sometimes, that is correct but this is bec. of a malfunction in our "programming". Acting illogically doesn't mean free will.
To reverse this discussion and look at it another way, what qualifies someone or something to have sentient rights? Self-awareness? Does that include the mentally retarded? Does that mean that we can/should enslave those who are NOT self-aware? Troubling thought.
Reproduction? Consumption for energy? Breathing? Fire does all these things. This basically degenerates into a discussion of: What is life? We haven't been able to answer that in millenia, I don't think we will answer it here.
Just like to be, or not to be! It is to live or not to live, as you have mentioned. Rights are for living things and it has nothing to do with IQ. Even self awareness is for living things. You have to be organic to be aware of yourself. Other kinds or awareness is nothing different from a digital sensor. It just digitizes the data and may record it, for you, for us. That's all. Droids are tools designed for humans to use them.
But, even a human body is a machine. A machine that is fighting illnesses, fully automated cage to keep your living energy(soul, if you like) in this world we know. If this machine is broken than we can no longer exist on the know existence. This body is like a space suit. We need that suit to live, to continue our existence here. But that "life energy" changes everything. I believe that energy is limitless, but this body has limits and very fragile. If a time comes that you can regenerate all the tissues of your body, than we can talk about immortality. Even then an explosion can tear your body apart and you will have to leave that suit. A droid does not need this. It's every part can be replaced, even its software and memory. Does your car or your refrigerator has rights? So why should a droid have one? (That's a general question