I guess this is how I see the connection working. Empathy links self-interest to altruism. If you feel pain yourself when another feels pain, then you do not want others to feel pain, because it hurts you. This would motivate you to avoid harming others, because that would cause you pain, and to protect others from harms committed by a third party, because that will similarly protect you from pain. If you feel pleasure when another feels pain, then you would want others (in general, or a smaller group, or an individual) to feel pain, because it pleases you. This would motivate you to harm others, and to allow third parties to harm them.
We can trust others generally to do those things which are good for them, so if they have a selfish reason (empathy) to be altruistic, we feel it is safe to trust them to be altruistic. But when they lack a selfish reason for altruism, or worse, have a selfish reason to harm others, we do not feel safe trusting them to be altruistic. So we feel safe around people who exhibit empathy, and we do not feel safe around people who show a lack of it (aka psychopaths).
Whether or not an instance of schadenfreude is itself harmful, it demonstrates this lack of empathy, and the conclusion is that the one having a laugh at another's misfortune is unreliably altruistic. The injured party's pain did not inspire suffering in the gloater, and if I am injured, my pain may not inspire suffering in them either. I want others to suffer with me, so that they will be motivated to help me, so I will reward behavior showing empathy and discourage behavior that does not show it.
Does that make any more sense? (Mind you, I'm not positive I understand your point properly to begin with, so this may not be addressing it at all)
Ajax
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