The principal sources for Darius' reign are the Behistun Inscription, presumably written by Darius himself, tablets found at Persepolis which are largely concerned with finances, and Herodotus' "Histories".

It is from Herodotus, and not Darius himself, that we get reports of the madness of Cambyses. There are good reasons to accept that Herodotus' Egyptian sources for Cambyses were hostile to the Persians in general and this king in particular. From no other source do we get reports of Cambyses that corroborate Herodotus' accounts of erratic behaviour. The story of the killing of the Apis Bull, for example, is contradicted by Egyptian sources recording the births and deaths of these sacred animals.

Darius himself tells us the story of the "false Smerdis", Gaumata, in the Behistun Inscrption, recounting how he killed a usurper to the throne who was disliked by all.

On the killing of the "false Smerdis" Herodotus and the Behistun Inscription appear to agree on the whole, but this may be due to Herdotus using sources for this tale that amount to Persian propaganda from supporters of Darius.

So it seems to me that to accept the story of the killing of the false Smirdis is more or less to take Darius' word for it, and the word of his partisans. That Darius may have lied to make his ascension to the throne appear to be something other than plain old usurpation (his claims to linage from the Achaemenids in the Behistun notwithstanding) should be accepted as quite possible.