That's Zero's fault.
Suzaku: YOU KILLED EUPHIE!
Zero: lol, forget about her and help me get Nunnally.
Suzaku: FUCK YOU.
Zero: Huh, I thought that would have worked.
That being said, Suzaku didn't execute Lelouch, but instead brought him to the Emperor. If Suzaku thought Lelouch was genuinely repentant, he would accept it, though probably not forgive him. Euphie wouldn't want Suzaku to kill Lelouch, anyway.