Lets see:
Itachi had to kill his parents with his own two hands.
Had to kill most of his clan members with his own two hands.
Watched his best friend commit suicide.
Got blamed for killing his best friend.
Had to leave the village he loved forever.
Had to participate in the killing of more innocent people.
Had to live with the fact that the people who he saved by killing his own familly, despised him from the bottom of their hearts. Including his own beloved little brother.
Had to live with a terminal sickness for who knows how long.
Ask yourself, was there really anyone who has suffered even close to as much as he has?
Let's see what Sasuke has been through
1. At age 7, found the dead bodies of his entire family
2. Forced to witness his brother slaughter their family over and over for 24 hours straight. All before he was ten by the way
3. Shunned by his father, feeling no love from him, then his father dying when he finally started to feel it
4. Lived all of his life alone, and with no real social interaction, resulting in no real friendships
5. Once he finally started to feel real friendships, his brother returns and places him in a coma where he saw his clan get slaughtered AGAIN for like 3 days straight
6. Had to live with the resulting psychological trauam
7. After abandoning all of his bonds, relationships and friendships to kill Itachi, he realized this actually achieved nothing. Sasuke sacrificed his childhood, the chances to make real friends and develop close and true bonds, to carry out an execution on someone who really loved him. Imagine if you sacrificed everything you have, your family, friends, jobs, life, only to find out what you sacrificed it for was really a bad thing you did