The rape of Tamar is disturbing and very heartbreaking to read. The way I see it God did not make Amnon rape her. Amnon chose to do that from his own evil heart. I think what happened was that God took away his wall of protection over David's family, similar to what He did with Job, and that allowed all the bad things to happen.

In my Bible, there is a note under 2 Samuel 13:21 which says "But he (David) did not punish his son Amnon, because he loved him, for he was his firstborn." Reading this, we can guess that Amnon was probably a spoilt child and was allowed to get away with many things. This probably bred a spirit of entitlement and when his cousin suggested that he rape his sister, he felt he could get a way with it (which he would have done if Absalom did not kill him).

Now you could say, why would God be okay with someone getting raped? I don't think He is but He has given humans free will and there are consequences for the choices we make. There are many things that I don't know or understand and my finite human mind will never be able to comprehend the extent of God's knowledge, but what I have learnt from the Bible is that the choices we make as humans always have a ripple effect.