This claim is completely wrong and it comes from non Muslims who put allegations on the prophets that they committed sins.
Musa was a prophet and all prophets are Infallible.
Musa killed that man because he was defending a believer when that man wanted to kill him. The believer called for help from Musa and Musa helped him by killing his enemy who was entitled for this punishment.
Any narration which contradicts the infallibility of any prophet or Imam must be carefully assessed and understood according to the teachings of Quran and authentic Hadeeth.