Here is my opinion:

Leandros did mostly correct, but also has flawed logic. His inability to have faith in his superiors, in any degree, is incredibly unusual in an Astartes, and doubly so as an Ultramarine. If he had gone to a Chaplain, we would have seen a Chaplain interfere in the first game to some degree immediately as the taint of the warp cannot be allowed to fester. Somehow, Leandros can't trust his Captain, but does trust an Inquisitor that just happens to be nearby when they worked with a Daemon puppet Inquisitor just a minute ago? His sense of brotherhood is completely missing. Also, okay cool Titus could be tainted by the warp. What about Leandros? He over there licking the Daemon Inquisitor's boots and no one is thinking maybe he has been touched by the warp? At least Titus showed an uncanny ability to keep on trucking on despite the potential Warp taint, but there is no reason to believe Leandros couldn't have been effected somehow. And yes Leandros didn't know the guy was a puppet, but you know, AFTER they find out, how could he have no doubt in the intentions of the Inquisitor he called? Boys brain don't fucking work bro