Rashi on Numbers 35:22/רש"י על במדבר ל״ה:כ״ב

1בפתע means BY ACCIDENT. In the Targum it is translated by בתכיף (in close proximity), meaning that he was very close to him and so had no time to be careful about him (i.e. to be on his guard not to kill him; see Rashi on Makkot 7b).