Rashi on Deuteronomy 19:11/רש"י על דברים י״ט:י״א

1וכי יהיה איש שנא לרעהו BUT IF A MAN HATE HIS FELLOW [AND LIE IN WAIT FOR HIM] — It is through his hatred that he comes to such a point as to “lie in wait for him”. From here they (the Rabbis) derived their statement: If a man transgresses a light command he will in the end transgress a weighty command; — because he transgressed the command (Leviticus 19:17) “Thou shalt not hate thy brother in thine heart”, he will in the end come to such a point as to shed blood. It is for this reason that it is stated here, apparently redundantly, “but if a man hate his fellow [and lie in wait for him]”, for it ought to have written only: “But if a man rise up and lie in wait for his fellow and smite him mortally” (Sifrei Devarim 186:2).