The only law on the books that ends in execution by the 24th century is daytriping to Talos IV.
Any order that ends in murder is therefore not on the books.
I suppose if you shot a volley from a phaser battery at another ship and 900 people die, it's just collateral that those 900 people died, and the Captain didn't order her weapons officer to kill anyone because there's a reasonable assumption that the opposing vessel can surrender or retreat before shit gets real.
Kathryn spent a fraught night, a dozen hours awake, and almost as much having nightmares on the subject, worrying if she could kill Tuvix, and considering she spent so long contemplating, it's clear that the ship did not NEED Tuvok and Neelix for their skillsets they brought to the mission, it was about their right to live as people.
And even then it was because Tuvok had a family and Neelix had Kes,the final conclusion was about how much family these two crazy kids had that couldn't stomach the blend.
WouldT'Pel have accepted/frakked Tuvix had she been on site?
I just want to remind you all that Kieko O'Brien was regressed into the body of her ten year old self, and still tried to have sex with her 40 year old husband. If Miles really loved his wife, he would have stood his ground and submitted to her adult and mature needs.