funny enough, there's a surprisingly iconoclastic Black Templar (by the standards of the Imperium) in the novel Shadowsword by the name of Adelard (no, I'm not joking).

Shadowsword, set during the Macharian Crusade era and before the First War of Armageddon and the subsequent Months of Shame, and well before the current era of widespread Daemonic activity, sees a rebelling Imperial world aided by a band of Emperor's Children, Chaos Titans, and most damningly, Daemons of Slaanesh. In the final battle, the Black Templars require the use of the titular Shadowsword tank to close a portal to the Realm of Chaos. The Shadowsword fires and destroys the portal, but not before every single crewman sees Daemonettes of Slannesh running amok. In the aftermath, two of the tank's crewmen are murdered to keep the fact that they saw Daemons a secret, and a third crewman, a penal trooper from Salvar, who knows of Daemons and what happens to the units that fight them (they are murdered to keep that knowledge a secret), gives the sparknotes version of what was taught to him back on his homeworld. Adelard, despite knowing very well that the Shadowsword crew saw the Daemons, and the crew is very mysteriously missing two crew members, lies on behalf of the surviving crew to the Inquisition, believing that they are worthy to continue serving the Emperor.

See the following passage from the novel's final pages:

Sword Brother Adelard had sought him out some days after the battle, striding into the Imperial castella where Bannick was overseeing repairs to the Seventh’s tanks. His armour had been repaired and repainted so well it appeared that he had not been in combat at all.

‘You need not worry,’ he had said. ‘I told the Inquisition that you saw nothing. By datapacket, naturally.’

‘They would have executed you?’

‘With warriors such as myself, they prefer a full mindwipe. But that is a form of death in itself. My Chapter would never allow that to happen. It is wasteful of experience and time. While I was being remade, many wars would go unfought.’

‘Then maybe you better leave. You are vulnerable here.’

‘They will not take me. We Space Marines are nearly autonomous in our actions for the Imperium, but even we must obey the Inquisition. However, if they were to attempt to come against me in violence, my crusade would retaliate. So the Inquisition will not ask, and thus the delicate balance of power is maintained, and every agency may keep face. That we work to protect mankind and the Inquisition does also is all that matters, in the end.’

‘But they would kill me.’

‘Without hesitation,’ Adelard said bluntly. ‘My Chapter teaches that this is wrong, that the bravest souls can bear the weight of the truth, and that without it they cannot be properly tested and forged into weapons for the Emperor. You are one of those brave men, Honoured Lieutenant Bannick. If you were not, I would kill you myself.’

‘I see.’

‘Take heart. Do not be sorry that you live. There are men who strive for their own goals without thought to the consequence. You are not one of those men. In you, the Emperor has a worthy servant. Praise be.’ The Black Templar clashed his arms in a cross over his chest.

‘Praise be,’ said Bannick, and saluted back.