The leg movement did not look natural to me. Obviously, he didn't mean to fatally injure Johnson but that's literally what manslaughter is: the causing of a fatal injury without malicious intent/planning. Moving your leg like that and having a skate high enough to even possibly cut a player's neck is extremely reckless. If they can prove that he was in control of his body, there should be consequences. I doubt he'll face a serious sentence even if found guilty but the law is the law. I'm sure he feels guilty/awful about what happened but I'd presume most people arrested for manslaughter feel the same way when charges are pressed. I've seen a video where a guy who did time for manslaughter talked about how disturbing the experience was, you could tell he felt absolutely terrible about what happened.
I'm not gonna say he should be tossed in jail and we should throw away the key, but you don't just enforce the law for the sake of punishment, it's also to deter future situations where reckless behavior can endanger lives.
Once again, I only believe he should face charges if he genuinely didn't lift his skate accidentally. Even if he didn't intend to hurt him, if his actions were voluntarily avoidable, he is technically guilty of manslaughter.