Yes, that was an accident, but it was still an action of his. Additionally, he still used Euphie's actions to his advantage, as Zero.
Suzaku didn't want to turn Lelouch down either, but after he got to know Lelouch was Zero, plus the fact he had killed Euphie, there was not too much to do. He also used that opportunity to his advantage.
tl;dr: Both of them did what they had to.