Genesis 6 shows that men were ruining the earth. Jehovah sent Noah to help them turn around (2 Peter 2:5). So, they were doing really bad thing and God told them what they needed to do in order to survive. What makes you think that is murder? When a parent tell his child "Do not do that or you will be punished in this way" and that child ignores the parent's warning, would it help anyone if that parent did not do as he said he would do? What type of upbringing are parent's using on their kids today? How do kids behave nowadays? Is crime, disease and war getting worst or better? Now, compare things to Galatians 5:19-23. The fruitage of the spirit in the last 2 versus shows what happens when Jehovah takes care of his people. How is that compared to what humans are doing nowadays? Do you think that, since people are acting the same as they were in Noah's day, the outcome is going to be any different? You've been given your warning.