Or HaChaim on Exodus 32:28/אור החיים על שמות ל״ב:כ״ח

1 בני לוי, the members of the tribe of Levi, etc. The Torah did not say that all the Levites displayed this loyalty by rallying to Moses' call. We need to understand why this part of the verse had to be written at all and the Torah was not content with reporting the Levites' response to Moses' call in verse 26 when all of them are reported as rallying around their leader. Perhaps the Torah wanted to stress that no one except the Levites acted as executioners at the behest of Moses. Verse 26 in which all the Levites are reported as rallying around Moses would not have made it plain that Moses had charged only the Levites with the task of being executioners. That verse only told us that all the Levites responded to Moses' original call. We could have thought that some Israelites responded in addition to all of the Levites. The Torah had to make clear that only Levites actually carried out Moses' instructions. The emphasis in "the Levites did what Moses said" is not so much on the Levites who did not do so but on the absence of any of the members of other tribes being prepared to kill idolators.

2כדבר משה, in accordance with the word of Moses. The Torah emphasises that Moses' word alone sufficed to prompt those Levites to take such drastic action against the active idolators. It could also mean that they did so without delay as soon as Moses had finished giving them their instructions.