Okay okay, let me do my best to put thoughts into words here.
Lightning Dust is like Rainbow Dash if you took away all her feels, emotions, reactions, loyalty, experience, and what she's learned with the other mane 6, and adding more recklessness, wanton disregard for others, bad morals(it's okay as long as no one gets hurt), and quite a few other negative traits.
She's only like Dash if you don't understand Dash. There's a nice commentary on RD on the mane sub right now if you want to educate yourself.