Or HaChaim on Ki Sisa 33:3

English: 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). According 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 volu