It’s from English, not Sanskrit. More specifically, an archaic English feature, where you’d use “be” instead of “have” for the present tense, if the main verb denotes a change of state (such as “become”). Note how “I have become Death” sounds perfectly fine for modern readers.
Odds are that Oppenheimer was quoting either an archaic translation Bhagavad Gita, or one using archaic language (this is typical for religious texts).
Also give this a check. English used to follow similar rules for be/have as German does for sein/haben.
[Shameless community promotion: check !linguistics ! This sort of question would fit like a glove there.]