“Do you remember the boy? The one who smiled with too many teeth?”
“I heard - he was…lost… and his companion, driven by grief, he - they say he - became one of the Sith.“
They were both Dark Jedi (i.e., Jedi fallen to the dark side; Shisui has sociopathic tendencies and doesn’t really care about stuff that doesn’t amuse him, and Itachi
keeps him sane is like. For the fifth time this star cycle Shisui you cannot kill shopkeepers who don’t give you what you want. Shisui. Shisui put your lightsaber down.). Itachi lost Shisui to Light Jedi, and it broke Itachi; Madara took this chance to reclaim his wayward apprentice.
Itachi kills Madara, and takes Sasuke as his apprentice, knowing that as the cycle continues, his little brother will eventually kill him as the final test - but not before they find the Jedi who took Shisui from him, and slaughter them.
Dark Jedi are just concerned with themselves. Sith want to destroy everything in their path and rule the galaxy.