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... turbidum ingenium esse, et sibi incolumitatem rei publicae magna...

The dat. of agent, an extension of the dat. of the person interested, an alternative in Tacitus to the abl. with "ab": "the safety of the state was being upheld/protected by him with great care." Distinguish this usage from the predicate dat, "magnae curae sibi," "a great concern to him," which would imply no action (such as the murders of Sulla and Plautus) by Nero.