Joshua, along with other books in the bible, do have killing in them, but the difference is God was working with His people physically at that time and he was telling them what to do, and He had a purpose, but God didn't instruct us now to kill any nationalities. There was also a few people who died in the flood, but that doesn't mean that God wants us to drown people. Christ said for us to do as He did, and this was important enough for God to come in the flesh and show us personally.
Ok, I guess I'm going to have to quote scripture:
Luke chapter 19 verse 27, Jesus says, "But those mine enemies, which would not that I should reign over them, bring hither, and slay them before me.