The key part of murder is intent which is what I wish to explore. The question really is, did Data intend to 'kill' B-4?
In Nemesis, Data uploads his memories into B-4. Memory Alpha describes it as follows:
Data willingly copied all of his memories to B-4, hoping that the added experiences would help B-4 to expand beyond his original programming
This certainly isn't described as though he had the intent to kill him.
From the transcript of Nemesis:
DATA: I feel nothing, Geordi. It is my belief that with my memory engrams he will be able to function as a more complete individual.
LAFORGE: An individual more like you, you mean.
DATA: Yes.
LAFORGE: Maybe he's not supposed to be like you, Data. Maybe he's supposed to be exactly the way he is.
DATA: That might be so, but I believe he should have the opportunity to explore his potential.
Again, this sounds a lot to me as though Data didn't intend to necessarily overwrite B-4's memories, but to enhance him, to improve him as a fellow android to a level comparable to Data.
I don't think it was really ever Data's intention to 'kill' B-4 at all; it seems more to me his intentions were pure. After all, Data's memory banks aren't completely filled, so it was reasonable to think that B-4 may also have had sufficient room for storing Data's memories without wiping his own.
