God, being the creator of this world and mankind and the overseer of how he wants the world to progress has given a commandment to mankind to not kill their fellow humans. However, he has given mankind free will to also do as they please. Man can kill man if he so wishes. Capital punishment is even a man made solution to rid our societies of extreme wickedness and evil. God, as a perfectly knowledgeable and wise God with a perfect understanding of what is right and wrong, has determined to instigate capital punishment of the extreme wicked of this world and you look upon that as an act of a genocidal maniac. Can you imagine living in a society where many of them are ok with murdering, robbing, and doing evil to their fellow man? You would scream for justice and law to punish those who commit such acts. God has told us that in the days of Noah the earth was filled with violence.
Genesis 6:11-13
11 The earth also was corrupt before God, and the earth was filled with violence.
12 And God looked upon the earth, and, behold, it was corrupt; for all flesh had corrupted his way upon the earth.
13 And God said unto Noah, The end of all flesh is come before me; for the earth is filled with violence through them; and, behold, I will destroy them with the earth.
God, in order to raise a righteous people on the earth likely needed to execute capital punishment and rid the world of extreme violence. God is the ultimate authority over the earth. He is the ultimate governing body over all of us. As God, He has the right to execute a just judgment on us. Man, who is not all knowing and does not have all wisdom and understanding may sit back and point the finger at God and blame him for the things he does as being unjust and genocidal just shows how arrogant and lacking in knowledge they really are. Noah and his family were probably thankful to God for cleansing the earth and making it possible for righteousness to reign again on earth. In my mind I see pointing the finger at God and not the violent criminals is absurd.
As I have stated before, this life is really only a temporary existence in the eyes of God and all mankind and animals will again live within a body that is made immortal and will live forever and ever. God's punishment upon the wicked is not permanent but a just measure upon those who seek to make this temporal existence a hell for those others whom come here to have a more just and fair existence.
No, He will likely not punish you with mortal death for not believing in Him and calling him a Genocidal Maniac. What He might do is say to you that He cannot allow you into the kingdom of heaven because you have no respect for his justice nor his laws and thus you are not capable of living the law of the kingdom of heaven. You would then be placed into a kingdom where you are willing to abide by the law of a lower kingdom.