They were trying to get rid of the competition. Many Romans converted to Judaism, including Onkelos who was the nephew of an Emperor.

Of course, there is also the pesky issue of "If Jsus was the Jewish messiah why don't the Jews accept him?" question that made it seem logical to get rid of the problem rather than address the question.

סופי

And everything that Sarah tells you, listen to her voice. Bereshit (Genesis) 21:12