Or HaChaim on Deuteronomy 17:2/אור החיים על דברים י״ז:ב׳

1 כי ימצא בקרבך, "If there be found amongst you, etc." The reason the Torah adds the word בקרבך, "in your midst," is to place the burden of dealing with the sinner (idolator) on the local authorities. Jews in other cities are only charged with dealing with the idolator if the people of his own city have failed to do so. This is what is meant by the words באחד שעריך, "in one of your cities." Our sages in Ketuvot 45 make use of the expression שעריך both here and in verse 5 to rule that the execution of the idolator takes place where he performed the sin and not in the city where he was sentenced to death (assuming they are not the same). The reason that the verse concludes with "which the Lord your G'd is about to give to you," is a reminder that one of the reasons G'd gave us the Holy Land is that we observe the commandment to execute idolators.

2לעבר בריתו, "to violate His covenant." Moses means that it is important for the sinner to realise that he violated G'd's sacred covenant. This is the reason he cannot be executed unless he had been properly warned of the penalty in store if he proceeded with his intention to commit this sin. There is also a hint here that if the idolator told the people who warned him to desist that he had sworn an oath to serve this particular deity, that he must be reminded that such an oath is not valid as he had been obligated under a previous oath dating back to Mount Sinai not to engage in any form of idolatry. His execution would therefore be perfectly legal if he went ahead and ignored the warning.