I was just wondering if you can achieve oneness with the dark side. Darth Malgus was said to have merged with the Force after he killed his lover and it linked the act to Oneness. So is it possible?
I severely doubt it. Let me explain.
There are (generally) two "sides" to the Force, of course: the "dark" and "light." But in general, most people misunderstand how there are two sides, i.e., is there some greater entity called "the Force" which encompasses the two camps?
The answer to that question is no. "The Force" refers to "the light." In other words, the light side is the Force in its natural state. The dark side is simply a perversion of that natural state. See this answer to "Why did the Jedi want to bring balance to the Force?".
As such, the answer to your question is no. A dark-sided being cannot become one with the Force, or else he would be embracing the light. This is corroborated by the fact that in all canonical sources of oneness to date, it has only been achieved by light-sided Force users.
Here's a quote from Star Wars: Revenge of the Sith by Stover (p. 409, hardback, first edition):
Yoda did not move. "Eternal life. . ."
The ultimate goal of the Sith, yet they can never achieve it; it comes only by the release of self, not the exaltation of self. It comes through compassion, not greed. Love is the answer to the darkness.
The italicized text is that of Qui-Gon, who was canonically one with the Force at this point. And given that oneness is essentially becoming one with the Force without dying, it would seem that Qui-Gon is essentially saying that no Sith can ever achieve oneness.
Addressing your concern about Malgus: Here is the relevant passage from Deceived, the novel in which Malgus feels like he has achieved oneness:
Thinking of Eleena blew oxygen on the embers of his anger. In life, Eleena had been his weakness, a tool to be exploited by rivals. In death, she had become his strength, her memory the lens of his rage.
He resided in the calm eye of a storm of hate. Power churned around him, within him. He did not feel as if he were drawing on the Force, using it. He felt as if he were the Force, as if he had merged with it.
Malgus's feelings and reality are two separate matters.