IMHO one could say the following. Our Sages tell us that if the Children of Israel had spent one more day in Egypt they would have sunk to such a low spiritual level that they would not have been able to come and receive the Torah. It was therefore vital that Moshe not delay his mission for a moment.

Moshe knew that the outcome of his mission would be that the Children of Israel would come out of Egypt and receive the Torah; presumably he didn't want to leave his family out of this. But precisely because of the dangers involved in the journey is why he didn't circumcise his son. Hashem then threatened his life as a result and Zipporah came to the rescue just in time.