Platoa, there is one true way to God. He choses the best human being of any given time to convey that path. So prophets act upon God's guidance and DO NOT make mistakes. And they certainly do not contradict themselves. You mix up general laws with specific laws when you say "thou shalt not kill". There are many people who fought against the prophets and their messages because it was harming their interests. After all, revelation came in times of injustice and tyranny in order to re establish justice and equality. So a confrontation between God's followers and oponents is bound to happen at a certain point. When that happens "thou shalt not kill" is preceeded by "fight back against those who wage war against you".
As for how to worship God, He chose His prophet to set the example of how He wants us to worship Him. Yes, He knows what is inside us but He gives us a chance to prove it so that when we come to judgement we can backup our claim by what we did during our life. And in islam, belief isn#t just words, it must be followed by action.