Someone said that Moses was a mass murderer having killed thousands of of his own people out of anger and frustration. Where in the Bible does it state this, if anywhere?
This might be the instance referred to:
This could hardly be described as him killing people out of anger and frustration, though. The punishment for idolatry was (to be) execution. God himself considered their actions worthy of death, just before Moses came down the mountain:
After Moses asks God not to destroy the entire nation, he comes down the mountain and witnesses first-hand what they are doing. When Moses gives the command for those loyal to the LORD to start killing people, he even says that it is God's command: "Thus says the LORD." The deaths of these thousands come at the command of God, not from Moses, the man who just prior to this had pleaded with God not to destroy the entire nation.
The text doesn't provide us with any evidence to support the claim that Moses was any sort of mass murderer.
It doesn't and he didn't.
It does say he killed A man. And murder is murder whether you kill one or a thousand, so he was a murderer.
In my experience, the accusation of 'mass-murder' usually stems from a gross misinterpretation of the Exodus story caused by the presupposition that God was not involved.
If you rule out supernatural intervention, the plague episodes and the Red Sea drownings become folk tales. While nobody expects all the details of such legends to be real, they do assume such tales were based of a 'kernel of truth', and in this case that must have been a temperamental, vindictive and cruel Moses who terrorized the Egyptians in order to achieve his own political ends. It's amazing how twisted a story Exodus becomes if you do not assign God the role he claims for himself in the narrative.