it was murder. Janeway took Tuvix's life away without his consent.
His consent is not required. Tuvix is still a member of the crew, and wanted to be treated as such. There are many instances where a military commander may order a subordinate to sacrifice his or her own life. Consent is not required.
So if Janeway orders a crewmember to fire photon torpedos at a school bus of innocent children he has to follow orders?
No, because that would clearly be an illegal order.