There is a world of difference between an untrained Force-sensitive individual committing murder (which usually doesn't require the use of the Force) and a fully trained Sith Lord performing impressive feats such as emitting lightning or cheating death. Likewise on the light side -- an untrained Anakin Skywalker might be able to react quickly enough to pilot a pod-racer but that is not the same thing as Yoda lifting up an X-Wing with the Force.
Any "dilution" of the dark side caused by an untrained Force-sensitive individual committing murder would be so minor as to be inconsequential. Such an individual is no match for the power of a Sith Lord. Consider, for example, Hetton: a man who had collected Sith artifacts and documents to teach himself the ways of the dark side of the Force (he could even conjure some Force Lightning) but who had not received any formal training from a Sith and consequently proved himself no match against a trained Sith Lord. Hetton pledged himself to Darth Zannah (the apprentice of the reigning Darth Bane) and Zannah ordered him to help her defeat Darth Bane so that Zannah could become the new reigning Dark Lord of the Sith. In truth Zannah had merely set him up to be killed by Darth Bane so that Bane and Zannah could acquire Hetton's collected knowledge. When Hetton and five of his Force-sensitive bodyguard/assassins confronted Darth Bane, Hetton could do little more than beg Zannah for her help while he and his assassins were all easily dispatched by the Sith Lord:
[Darth Bane] whirled around to see a small man in his fifties [Hetton], clad all in black, standing on the far edge of the camp. In his hand was a green lightsaber, though it was obvious from the way he gripped it that he had never received any proper training in how to handle the exotic weapon.
At the man’s side was [Darth Zannah]; she had not drawn her lightsaber. Bane snarled in anger at her betrayal, his rising anger fueled by the chemicals the orbalisks were pumping into his system.
“Today is the day you die, Darth Bane,” the man said, charging forward to attack.
At the same time, the five red-robed figures rushed in from behind him. Bane spun and thrust his open palm toward them, lashing out with the power of the dark side. Like the Jedi and Sith, one of the first techniques Shadow Assassins learned was the creation of a Force barrier. Channeling their power, they could form a protective shield around themselves to negate the Force attacks of their enemies. But if an opponent was strong enough, a concentrated attack could still breach the barrier. Darth Bane, Dark Lord of the Sith, was definitely strong enough.
Two of the assassins were stopped in their tracks, knocked to the ground as if they had run into an invisible wall. Two more, weaker and less able to defend themselves against Bane’s power, were sent flying backward. Only the fifth was strong enough to resist the Sith Lord’s throw and continue his charge.
However, without his brethren at his side to harry and distract his foe, he found himself the sole focus of Bane’s wrath. Unable to defend against the savage sequence of lightsaber cuts and thrusts, he fell in a matter of seconds, half a dozen fatal wounds scored across his chest and face.
While the four remaining assassins regained their feet, Bane wheeled back to their leader. Wisely, the man in black had stopped his own charge and was gathering the Force. As Bane stepped toward him the man unleashed it in a single long, thin bolt of indigo lightning. Bane caught the blast with his lightsaber, the blade absorbing the energy. In retaliation he struck back with lightning of his own—a storm of a dozen bolts arcing in toward his target from all angles.
“Zannah!” the man shouted. “Do something!”
But Bane’s apprentice didn’t move. She merely stood off to the side, biding her time and observing the action.
The assassins fell on Bane again, but instead of repelling them with the Force, he allowed his body to become a conduit, turning himself into a physical manifestation of the dark side’s tumultuous power. As he spun like a whirlwind, his blade seemed to be everywhere at once: hacking, slashing, and slicing his enemies to ribbons.
All four assassins died in the attack, though one managed to land a single blow with his force pike before his throat was slit, the wound so deep it nearly severed his head. Fueled by rage and fury, Bane shrugged off the deadly electrical shock like a rancor shrugging off the bite of a venn-bug.
Once again he turned his attention to the man in black. Bane marched slowly toward him as his adversary stood frozen in place, paralyzed by the terrifying knowledge of his own imminent death.
“Zannah!” the man cried out to her again, holding his lightsaber vertically before him as if it were a talisman that could hold the approaching demon at bay. “Master! Help me!”
Bane chopped down with his own weapon, severing the man’s sword arm at the elbow. The man screamed and dropped to his knees. An instant later his voice went silent as Bane ran him through with a single hard thrust, the lightsaber entering his chest just below his heart and protruding a full half a meter out the back of his shoulder blade.
Darth Bane: Rule of Two, pp. 202-204
Even trained dark side users don't dilute the dark side of the Force too much. Indeed, the Sith often used trained dark side allies such as the Inquisitors and the Emperor's Hands as pawns to further their goals. As I explained in my answer to why the Inquisitors don't break the Rule of Two, the distinction between a Sith and an ordinary dark side user is that ordinary dark side users -- even when allied with the Sith -- do not have access to the secrets and full training of the Sith, nor do they participate in experiments to harness the full power of the dark side:
The two of them were in Sidious’s lair, a small rock-walled enclosure beneath the deepest of the Palace’s several sublevels that had once been an ancient Sith shrine. That the Jedi had raised their Temple over the shrine had for a thousand years been one of the most closely guarded secrets of those Sith Lords who had perpetuated and implemented the revenge strategy of the Jedi Order’s founders. Even the most powerful of Dark Side Adepts believed that shrines of the sort existed only on Sith worlds remote from Coruscant, and even the most powerful of the Jedi believed that the power inherent in the shrine had been neutralized and successfully capped. In truth, that power had seeped upward and outward since its entombment, infiltrating the hallways and rooms above, and weakening the Jedi Order much as the Sith Masters themselves had secretly infiltrated the corridors of political power and toppled the Republic.
Save for Sidious, no sentient being in close to five thousand years had set foot in the shrine. The room’s excavation and restoration had been carried out by machines under the supervision of 11-4D. Even Vader was unaware of the shrine’s existence. But it was here that they would one day work together the way Sidious and Plagueis had to coax from the dark side its final secrets. In the intervening years he had actually come to appreciate Plagueis for the planner and prophet he had been. Such perilous machinations required two Sith, one to serve as bait for the dark side, the other to be the vessel. Success would grant them the power to harness the full powers of the dark side, and allow them to rule for ten thousand years.
Tarkin, p. 101