Oh, you’ve seen his report, have you?
What, specifically, did he supposedly lie about, and how do you know he lied?
Cops are trained to retrieve any weapons dropped or displaced during an altercation so that bystanders can’t get their hands on them. It’s likely that when Slager saw that the cartridge was no longer in the taser, he simply dropped it because it was empty and not a threat. Then Slager appropriately continued to tend to Scott. He holstered the taser moments later.
Because his boss says he lied.
Cops are also trained to preserve evidence and apparently how to manufacture evidence.