First thing: it is normal that from time time you have negative thoughts about your dad. Everyone has. This hasn't made him sick.

Second: Cancer is not cancer. There are some very serious ones and some not so. Prostate cancer is something many older men get and in some cases it can stay a long time unchanged without getting dangerous. Many men die from other reasons although they have a prostate cancer. It happens so often that there are a lot of therapy options as well. So there is a high probability that your dad will be ok.

This is just some info to help you keep cool.

Your dad will need your love and in moments like this you see that the negative thoughts are only a temporary thing while the love between you is the underlying and stable feeling. That's the good thing in bad moments like that.