John Lightfoot on Matthew 5:43

[Thou shalt hate thine enemy.] Here those poisonous canons might be produced, whereby they are trained up in eternal hatred against the Gentiles, and against Israelites themselves, who do not, in every respect, walk with them in the same traditions and rites. Let this one example be instead of very many, which are to be met with everywhere: "The heretical Israelites, that is, they of Israel that worship idols, or who transgress, to provoke God: also Epicurean Israelites, that is, Israelites who deny the law and the prophets, are by precept to be slain, if any can slay them, and that openly; but if not openly, you may compass their death secretly, and by subtilty." And a little after (O! the extreme charity of the Jews towards the Gentiles); "But as to the Gentiles, with whom we have no war, and likewise to the shepherds of smaller cattle, and others of that sort, they do not so plot their death; but it is forbidden them to deliver them from death if they are in danger of it." For instance; "A Jew sees one of them fallen into the sea; let him by no means lift him out thence: for it is written, 'Thou shalt not rise up against the blood of thy neighbour': but this is not thy neighbour." And further; "An Israelite, who alone sees another Israelite transgressing, and admonisheth him, if he repents not, is bound to hate him."