Tuvix was a cad (a full blown Terry-Thomas cad). The situation gets re-scrambled in the later "Drone", when a transporter accident blends Seven's nanoprobes and the EMH's mobile emitter, with different results.
Yep, One was altruistic and willingly sacrificed himself to save others.
Tuvix was selfish and didn't.
But, still, selfishness is not a capital offense.