She killed a sentient being to bring back 2 of her friends.
To put that in contemporary terms... My SO needed a kidney transplant. Would I have killed someone to get him a kidney?
This. It was murder any way you cut it. Janeway had no right to terminate Tuvix against his will.
Sure, it's a numbers game, but it's Tuvix's decision to make. Does wanting to live at the expense of two others make him a coward? Quite possibly, but is cowardice a capital crime? Well I don't think so, but Janeway does.