Well you could easily argue that apart from Lily everything about Snape was dark and evil. Everything that Snape did that wasn't evil was for her. On the other hand Everything Itachi did was to protcect Sasuke, the leaf or his clan
snape was influenced by lily but he chose to help out people not named harry for little reason other than he had the chance to do some good like when he tried to blast a death eater before the death eater could kill george. and when he did everything he possibly could to adhere to dumbledore's orders and agree to kill him before the curse does. he offered to do the unbreakable vow to help out draco which again had nothing to do with lily. itachi did nothing to help or protect anyone but himself. the reasons why he wants to help people are loyalty to sasuke and the leaf but he did nothing to ACTUALLY help them besides mass genocide and mental torture which in the long run didnt do anything good. the butchering of uchiha clan is brutal insane and horrible. he slaughtered children and shoved a sword in his mothers chest. show me snape killing kids and shoving a sword in his mothers chest. he did nothing to even slow akatsuki down defeating the purpose of why he joined them in the first place and obeyed all of pains orders without question. even though members have gone against pain and killed their own partners scot free (kakuzu). the torture of sasuke and corrupting of his mind is injust, unjustifiable and unbelievably brutal. he didnt just torture him, he taunted him, made him feel like shit, broke his wrist, ordered him to murder people for power and slammed his head against a wall. again show me snape doing the equivalent or worse.
itachi tried to enslave a (as far as he knows) innocent boy to serve danzo, two old crotetchy hags and a drunk for the rest of his life. itachi tortured and ordered the attempted killing of kakashi for no reason at all. itachi stood back and did nothing as his partner was inches away from cutting narutos legs off. again i point you to snape trying to stop his fellow death eater from blowing george away. and as for itachi acting for the leafs benefit, taking down their greatest fighter at the time and putting him in a coma then ordering the attempted murder of the next two strongest, is not helping anything. letting orochimaru go when he could have killed him in self defence is not helping anyone especially the thousands of victims orochimaru butchered.
threatening konoha with high treason for one little boys sake is damaging and traitorous
withholding vital intel about akatsuki and the uchiha clan from hiruzen is damaging and traitorous
and itachi didnt have an ounce of regret or empathy for naruto and the whole ordering his execution at sasukes hands, even smiling at the thought of forcing two friends to battle to the death
ensured sasuke would believe tobis story and placed amaterasu in him. without that, showinghis tears to sasuke post genocide and apologising to sasuke, sasuke would have dismissed tobis story as bullshit