Was Euphemia's mother ever mentioned? It is possible that she died or something and that Cornelia raised Euphemia for several years. And she lost Marianne, who was a friend to her and possibly like a mother to her.
Euphemia's fahter didnt want her and treated her like a political tool.
Her long-lost brother treacherously murdered her.
She was forced to go into battle at Narita to make sure that Cornelia did not massacre the JLF and/or the Narita towns people.
her father sent nunally away
She often felt alone when she saw how everybody was willing to do evil things.
She was lead to believe that Nunnaly and Lelouch were dead, which tore her apart for seven years - much longer than Lelouch believed that Nunnally was dead.
She discovered that her brother was actually a psycho that really didnt care much for her anyway
The Japanese hated her despite the fact the her last words were hopes for their happiness.
Here is just one character, Euphemia, who seems to have suffered about as much as Lelouch did. But it took a geass command to make a mass murderer out of her. As long as she had control of herself, she didn't let her suffering turn her vengeful - unlike Lelouch.