Why is the Russian still alive in Heart Of Darkness?
That depends on who or what you think is going to kill him, and at what point.
When we first meet him, he's been a disciple of Kurtz's for some time (Kurtz "expanded his mind"), and thus under Kurtz's protection--if he even needs any protection. Kurtz isn't interested in killing him because he isn't one of the "brutes" Kurtz has decided need to be killed. He apparently doesn't contradict Kurtz, either, thus giving Kurtz no reason to hurt him ("You don't talk to that man--you listen to him"). Having decided that Kurtz was some sort of god (and being a white man), the Russian was safe from him.
The Russian seems quite harmless, and even fairly pleasant, if not slightly mad. The one man he owed anything to (for borrowing guns to go upriver alone), he sent some ivory in payment.
As Marlow is talking with the Russian after Kurtz's death, he says that "He suspected there was an active ill-will towards him on the part of these white men that—"
Marlow replies: "You are right.... The manager thinks you ought to be hanged."
(The manager, who Marlow had met earlier in the trip, had referred to "a pestilential fellow, snapping ivory from the natives" who hung out with Kurtz. He added, "We will not be free from unfair competition till one of these fellows is hanged for an example.")
Upon hearing this, the Russian says, "I had better get out of the way quietly." He confirms that he has friends among the natives and will be fine, then bums some cartridges and tobacco from Marlow, disappearing into the jungle.