The problem with this argument is that it has virtually no ceiling. You start with mass murderers, proceed with torturers, sadists, animal abusers, child abusers, rapists, and from abjection to abjection you end up justifying to yourself not forgiving your neighbour for letting his dog poop in your yard.
I think the greater the evil, the harder it is, most understandably, to forgive. But it remains a spiritual duty.
Although, I don't think forgiveness is a legal action. Breaking the law should always have legal repercussions. I take it more as a safeguard against hatred, a security net not to freefall towards resentment, hatred and ultimately a realm where no redemption is ever possible.
"Never forgive, never forget" sounds cool but from a spiritual standpoint, it's not much less dehumanizing than horrors such as mass murders.