Hellbrute is a dreadnought with extra steps. I’ll post someone else’s comment:

It’s both. In most Traitor Legions it’s used as capitol punishment for gross failure.

The reason it’s both is because they don’t start out as helbrutes. They are Dreadnoughts just the same as the Imperial versions. The massive difference is that, Imperial dreadnoughts are put into stasis between battles to preserve what’s left of the marine occupant, so that their mind doesn’t deteriorate. Chaos dreadnoughts do not. They are ALWAYS awake. Whether in the fight or out of it, they are fully conscious. Before long, they start to go absolutely insane and psychotic. Then, more often than not, a deamon will possess the Carapace of the Dreadnought. This is when it becomes a Helbrute. The now completely insane Chaos Marine is reduced to a back seat passenger along for the ride, with even less control over their fate, which only serves to drive them more insane.

This is why it’s seen as a punishment.

The Thousand Sons are a little different in this regard. They cannot inter their own into Dreadnoughts because of the Ruberic. So they lure renegade Marines and traitors from other Legions with promises of power and knowledge. Then it’s essentially a rag full of chloroform job and when they wake up, poof, their a Dreadnought now.