Nihilus/Traya's Drain Is Not An Instakill
Complete no-limits fallacy to believe that this is the case to begin with, but should anyone require the proof, I just posted the following in another thread, explaining how Nihilus/Traya's Drain works and how it isn't an instakill:
It's not Severing in the sense of your average Sever Force, but it's Severing you from life itself as well. This is mentioned more than once in KotOR II:
"It is a technique that is almost as old as the Sith themselves... it is a means of severing connections between life, the Force, and feeding upon the death it causes."-- Knights of the Old Republic II: The Sith Lords
"I was the only living thing remaining on the planet of Katarr... and my life, my agony, was a flicker in the darkness that was the planet. All that I had been connected to had been severed."-- Knights of the Old Republic II: The Sith Lords
There's a reason why Meetra wasn't Draining people when they were alive, but only doing so once they died - because this technique feeds on death, not life. It requires you to cause death by Severing connections between life and the Force, and then feeding on the death(s) that occurs as a consequence. This is also why this ability is distinct from other versions of Drain displayed in the mythos.
It stands to reason that Nihilus can't do this Severing against beings who are more powerful than him. The no-limits fallacy argument is further disproven by the game, since Traya survived Nihilus' Drain despite him Draining to kill.
"There is more. Darth Traya yet lives. You did not kill her, as you assumed."-- Knights of the Old Republic II: The Sith Lords
And the case with Traya also links back to what I said above, since the KotORCG claims that her powers were drained by her apprentices, yet she claims in KotOR II that she was "stripped of her powers". It was an incomplete process of the quotes I provided above.
We can add to this the fact that the Assassins use an identical technique to Nihilus, yet they never instakilled anyone. This is because they're not strong enough to do so, again proving that it isn't the technique that kills but the power of the user:
"The sect of assassins that chase you feed on the Force... what he does is simply the pinnacle of what they could achieve, in time."-- Knights of the Old Republic II: The Sith Lords
To all the people who think that Nihilus (or even Traya) can instakill anybody with their Drain bar Surik, you're dead wrong, as per KotOR II. They can do it, if they're strong enough to completely carry out the process I explained above. If they can't do that, their Drain will only have a partial effect, similar to the Assassins. They'll still get stronger, and their opponents presumably weaker, but nothing more - certainly not an instakill.