Malice is not needed for cruelty.
And her actions with Cadence are absolutely malicious.
True on the first count, but I'd categorise her actions towards Cadence as caused by a reasonable level of emotional
control for her age, combined with ignorance about the emotions/actions of others involved (a subset of emotional
maturity), leading to emotional instability (from lack of perceived support from Celestia) and fluctuations (from incorrect views of events as more emotionally salient than the reality) beyond what most people need to keep in check.
Just because an action has negative consequences doesn't mean there are sufficiently-better
viable alternatives to call that action cruel, so I'd disagree with that characterisation.
Addressing the general point re: the appropriateness of punishment (both positive (impede/chastise) and negative (ignore/isolate)) for ignorance-induced cruelty:
If we use non-malicious cruelty as a threshold to decrement "worthiness" and justify punishment, we'll be punishing absolutely everyone (not taking minor low-cost actions in some cases causes thousands/millions of deaths through inefficient resource allocation, IRL, which is definitely non-malicious ignorance) and getting very little beneficial behaviour change out of it (ignorance can't be resolved just by adding negative incentives, since the subject lacks the mental pathways to actua
lly do what you're expecting them to do; sufficiently powerful incentives on each step of the process can admittedly overcome this, eg the American criminal justice system, but are impractical and wasteful compared to targeted education and weaker incentives).
It just doesn't seem like a good moral standard to implement social cooperation purely through negative reinforcement, and if we treat actions as punishment-worthy independently of cognition/intention/history, that's essentially what we end up doing.