If you kill a family of five in cold blood as a non-Christian, but come to believe in Jesus Christ as your LORD and Savior, then you can ask God to forgive your sins, and you will be forgiven. You will not go to Hell.
This should answer your question.
But it is important to recognize that not all killing is sin. A US Soldier killing a Muslim ISIS soldier is not sin. In fact, it is the US Soldier’s duty to kill Muslim ISIS in a war between USA and ISIS.
The 10 Commandments forbids murder (unjust killing), but the Bible clearly shows that God allows killing, such as for soldiers. Sometimes, God commands killing. Read through the Book of Joshua and the Book of Judges in the Holy Bible.
For instance, opposing homosexuality and gay marriage is illegal in the United States of America. If you are a true Christian and oppose homosexuality, then you can go to prison after being arrested by the police. United Sates of America has chosen to stand against God and the Bible on the issue of homosexuality, unfortunately.
But it is important for Christians to always remember that while you keep God’s law, you may end up breaking your nation’s law. You have to be cunning enough to evade law enforcement, when you are serving Christ and honoring Christ with your body and skill sets.