Apparently this is Halacha from Rambam, as written in Avodah Kochavim:
http://www.chabad.org/library/article_cdo/aid/912369/jewish/Chapter-Ten.htmHalacha 1
We may not draw up a covenant with idolaters which will establish peace between them [and us] and yet allow them to worship idols, as [Deuteronomy 7:2] states: "Do not establish a covenant with them." Rather, they must renounce their [idol] worship or be slain. It is forbidden to have mercy upon them, as [Deuteronomy, ibid.] states: "Do not be gracious to them."
Accordingly, if we see an idolater being swept away or drowning in the river, we should not help him. If we see that his life is in danger, we should not save him. It is, however, forbidden to cause one of them to sink or push him into a pit or the like, since he is not waging war against us.
To whom do the above apply? To gentiles. It is a mitzvah, however, to eradicate Jewish traitors, minnim, and apikorsim, and to cause them to descend to the pit of destruction, since they cause difficulty to the Jews and sway the people away from God.
So it is correct... He also did say that we must assume a non-Jew is not a Bnei Noach and thus assume that he is liable to the death penalty...