Radak: It means Ger v'Yasom. This is like "Shemesh Yare'ach" (Chabakuk 3:11); the prefix Vov (for 'and') is omitted. The same applies to others; these are mentioned because they are often oppressed due to their weakness.

Malbim: If you will do Mishpat (verse 5), all the more so you will not oppress an orphan, convert or widow, and all the more so not murder, and all the more so not serve idolatry. I.e. one who does good because he himself is good, and not due to fear of punishment, all the more so he will refrain from evil knowing that it will damage him.

Radak: It is evil everywhere. It is worse here, for it is the place of Shechinah.

Radak: Following other gods is bad for you. When you do so, you fall in front of your enemies.