Is it possible that he didn’t think anyone would be killed—that they would get Cipro and be fine, since there were warnings enclosed and it wasn’t known that the stuff could permeate the envelopes?
Some of the "evidence" against him was awfully shaky. For instance, the FBI released emails in 2008 in which Ivins describes being treated for mental problems--nevermind the fact that anyone hounded by the FBI the way he was might develop some mental problems. Another piece of "evidence" is that he was working late laboratory hours. Oh, heaven forbid that a scientist work late in the lab, it MUST be a sign that they're up to no good (says sarcastically the person who, as a grad student, often worked til 10 pm or later, and came in on weekends and holidays as well).