it isn't a deterrent,
It is cheaper to let them rot in prison for life,
nobody wants to make the drugs involved for the 'humane way' so it is really difficult to obtain enough where it is used,
it is fundamentally inhumane to kill someone that knows it's coming (mental torture),
risk of executing an innocent, and as already stated
it is hypocritical to kill someone for killing.
All they do is increase taxes though. /s