HAHAHAHAH. both were mindraping spiteful thugs who dabbled in evil techniques, and made the people they are supposed to protect, unbearably miserable and weak.
Snape lived a lie, hid his best nature, lived under constant threat .. all for a long lost love who friendzoned, err no, enemyzoned him (Obito should learn a thing or two from Snape) .... very few can give a greater sacrefice.
snape was a spiteful thug who regularly made it his mission to make everyone in hogwarts except his beloved slytherins miserable.
he tried to expell harry, mindrape him, and regulalrly made sur ehe would do nothing at all when harry is in terrible danger eg the fight vs a dragon.
being a spy for dumbeldore does not excuse being a bullying douchebag.
They have different personalities, but they both decided to give everything to the person they loved most
lol they are nothing alike. itachi is a pure evil sadistic monster who murdered hundreds of poeple, and tortured his brother to insanity. snape is a petty spiteful ass.
1) They both were spies inside of a criminal orginization
snape gave useful information. itahci betrayed konoha, hoarded information and left konoha clueless about everything
2) They both were protecting someone who hated them and wanted to kill them
neither did that. snape hated harry and did everything he could to make sure harry was alone, ostracised and weak, even attempting to expell harry for something he only suspected harry was involved in. snape had hate for dman near everyone except lily and his fellow slytherin thugs.
itachi murdered his family and tortured/savagely beat sasuke into a coma and later on, insanity.
4) They sacrificed everything for love
they sacrficed innocent people for their own selfish goals. in itachis case it means killing hundreds of people so an evil dictatorship can survive. for snape it means being a horrible selfish bully to harry and abandoning him when he needs help the most
5)Their sacrifices were not known until after their deaths
and hopefully will be hated like the horrible people they are