|So if I decided to kill thousands of people then just ask God for forgiveness I'm good?|
Yes. However, the thing is, if you are the kind of person to kill thousands of people, will you honestly be asking God for fogiveness? Or, as a Christian, would you have preferred to not do the action in the first place? Of course, you do get some times when people have committed a crime, and converted to Christianity, where they asked for forgiveness. Probably the most well known examples of this are Saul / Paul from the Bible, who was persecuting Christians, God made him realize what he was doing and then became a preacher, spreading the word of God. Another example is of John Newton (the guy who wrote 'Amazing Grace'), who actually used to be a slave trader.
|Well if he does, and you repeat it again, you can't expect him to forgive you the third time.|
Yes, you can. Due to God's grace, he will forgive you over and over again. Otherwise, seriously, will anyone still be 'good enough' before a few minutes in their life have passed? Jesus says in the Bible somewhere something like 'Don't just forgive him 7 times, but more than 70 times 7 times.'
And as for 'I don't care, the Bible says its wrong so no one should do it', that is true. Just because we don't always manage to do what the Bible says doesn't mean that it should become right.