Was Tuvix a sentient being?
I think most of us would agree he was, and as such being on a Federation ship was under the protection of Federation Law.
What offense did he commit?
I think most of us would agree he didn't commit any offense.
Was General Order 4 violated?
As far as we know violting that order is the only offense which carries the death penalty.
Tuvix can not be held responsible for how he came into being, it wasn't his fault that his creation resulted from the merger of Neelix and Tuvok. We generally don't punish people for things over which they had no control.
So what crime did Tuvix commit to warrant the cestation of his existance? Wanting to live is not a crime.