The definition of malevolent is having or showing a wish to do evil to others. If being able but not doing something make you malevolent, then isn't everybody malevolent? If you see a homeless guy on the street asking for money and you are able to give him $1, but not willing, does that mean you want to do evil to him? Right now there are starving children somewhere in the world and you have the ability to go search for the many organizations on google and make a donation to feed them, but are you willing to? I guess that means you're malevolent and want to see children starve, right? I mean according to Epicurus's logic, thats exactly what it means. You want to hurt children and see them starve because you're not willing to go donate $5 right now.
...and now you can see how Epicurus's quote falls apart right there by not using the correct definition of malevolent.