I do not think he was insane, that is an excuse used now a days to escape punishment. Then of course we can see the difference between then and now. At that time punishment was hanging so it tells you that these sort of incident (murder) were not common. Now a days those punishments do not exist, so just look around you.
Now towards God factor. What few people think gives the idea that they want God to intervene into ever thing, even if a worker at local McDonalds is going to sprinkle a bit too much salt on fries then there and then God should appear next to the fryer and tell him off otherwise he may give many people hypertension and death due to that. So God is not the character from TV comedy show
my hero.
http://www.bbc.co.uk/comedy/myhero/index.shtml
I was watching a programme on animal planet other day when the camera woman (presenter) saw a lion heading towards a lost cub of a cheetah. She was worried that the lion would probably kill the cub and said that she felt like driving towards lion to scare him off but did not do that. Because that is part of rules and regulations there that you should not intervene because this is all natural process.
God has created people and given them free will and also has explained what is good and what is bad through his messengers. From there on it is a
test. Had he wished to create every one so perfect and without free will then why did he need human. He had perfect example in angels.