Was Musa sinless?

I have read this question and I have also studied in school that Prophets are sinless. Today I read in the Quran that Moosa killed a man. Here are the verses that describe the scene. I read at another place that he 'accidentally' killed him. I don't find it accidental myself (Musa struck him with his fist and he died.) Are Prophets sinless? Even if he hit the guy with his fist, its considered a sin.