Actually, my answer was that those who disobey God deserve to be executed, including myself.
So, rephrasing your question slightly, how is it then that anyone "should" be executed when no one is allowed to kill? The short answer is that the prohibition against killing humans is a general rule or principle that holds for all humans and all animals (Genesis 9:5-6) unless God specifically commands otherwise.
Christian moral "oughts" can be roughly broken down into three categories.
The first category, the overarching rule, the one that applies to all moral agents (absolutely everyone who has a moral code that they ought to follow), the one rule that is never to be violated under any circumstances, by anyone, is "obey God" (Mark 12:30; John 14:15). This singular moral law governs all humans, animals, angels and demons. Even God, by His own desire and authority, continuously obeys Himself (2 Timothy 2:13; Philippians 2:8). Absolutely everyone ought to obey this rule absolutely all the time.
The second category that applies to humans would be general rules of behavior, given to us by God. Most of the Ten Commandments would fall into this category. In general, humans aren't to kill, to lie, to steal, etc.
The third category would be situation-specific or person-specific commands, also given to us by God. For example, children ought to obey their parents (Colossians 3:20). Adults do not need to do this.
Sometimes, these situation-specific commands are actually exceptions to the general principles in the second category above. For example, no one should kill a human, but if that human has killed a human, then other humans are morally obligated to kill him (Genesis 9:5-6). Of course, the Bible also provides us with exceptions to this exception (Numbers 35:25).
If the penalty for the crime has already been paid, of course, then it need not be paid twice. This is the case for all who put their trust in Jesus Christ (1 John 4:10; Romans 10:9; Colossians 2:14).
However, simply because we can be forgiven for our crimes doesn't make them any less heinous. It doesn't suddenly make it okay to sin. Our own individual crimes should be among the most reprehensible things imaginable to us (Romans 6:1-18, 7:22-24; 1 John 2:1), and from this attitude, repentance will follow (Acts 8:22, 17:30).
In sin, all die, but in Christ, we can be made alive (1 Corinthians 15:22; 1 Peter 3:18).