I dont understand. God says in the 10 commandments do not kill. But then God contradicts himself saying kill jews that sin in certain cases and kill gentiles in certain cases and kill animals as sacrifices...

The difference is obvious: one isn't allowed to kill the innocent but the guilty people should be deterred and punished (even that was usually not carried out, but it's written in order to show the seriousness and deter, Regarding animals, in fact it's only allowed when it's for a higher need, e.g. man's needs, but otherwise it's generally prohibited.