If there's some way to free them from their corruption, then that needs to take priority over all other alternatives. If an Epic is already reformed or is known to be fighting corruption, they should not be killed or otherwise targeted.
If there is an Epic like Fortuity who kills regularly and shows not the slightest sign of corruption, then I feel it is ethical to intercede on behalf of his future victims and put an end to him.
Even the ones who were regular killers should not be held accountable for their actions, so to speak. While their assassinations might be necessary they are clearly not responsible for their behavior, as extreme cases like Phaedrus' dramatic turn towards evil prove. Killing might be necessarily, but ethically Epics aren't much more responsible for their crimes than the victims of a virus that turns them into zombies.