Magearna isn't opposed to other Pokémons' pain and suffering. She craves it. Here's an excerpt from the website describing Magearna:

>Magearna has the power to perceive the emotions, thoughts, and feelings of other Pokémon. If a Pokémon is injured, Magearna will feel the other’s pain and suffering and will try as hard as it can to save that Pokémon.

However, her ability, Soul Heart, is the opposite: she gets stronger as Pokémon faint. And not just the opponents Pokémon, but her allies as well. If used competitively, one would want to faint as many Pokémon as quickly as possible, to boost her special attack in the process. Simply put, feelings, pain, and suffering from fellow Pokémon fainting gets Magearna pumped.