Or HaChaim on Exodus 33:3/אור החיים על שמות ל״ג:ג׳

1 פן אכלך בדרך, "lest I consume you on the way." There is no comparison between someone who angers his teacher while in his teacher's presence to someone who takes advantage of the teacher's absence to anger him. בדרך, "on the way." This is a hint that the announcement that "only" an angel will accompany the Jewish people would not apply at all times and in all circumstances. When Israel would be in a city, i.e. in a relatively safe environment, G'd Himself would be near them (compare Isaiah 26,1).

2According to the Kabbalists the desert is the home of Samael who succeeded on several occasions to overpower and kill Jews who lacked G'd's protection due to their share in the sin. The Zohar volume 2 page 157 expands on that theme when commenting on the words המדבר הגדול והנורא in Deut. 1,17. It uses that verse as proof of how dangerous it is to be בדרך, "on the way."