Love the sinner, abhor the sin.
I, the Lord, will forgive whom I will forgive, but of you it is required to forgive all men.
Neither do I condemn thee: go and sin no more.
We are to forgive all men no matter what they may have done. It frees up the soul to move forward and progress. Being hung up on things done to us stops us in our spiritual, emotional, and mental progression. So forgiving is actually important for us even more than it is for the sinner. For most of us, we don't even have to think about forgiving Hitler, because he's done nothing to us, personally to forgive. Whether he will obtain forgiveness in the eternities is, fortunately, not up to me.
As for forgiving Satan, we have been given examples of how to deal with him and his kind over and over and over. "Satan, get thee behind me." Don't even give him or his minions a chance to sway you. Fortunately, he, being the Master of all Lies, will never outright try to hurt you, but, instead, to convince you to betray yourself. We do not need to forgive him because he will not be allowed to do anything against us that we cannot overcome with our own Faith and God's help. Even in the case of Joseph Smith, when he physically attacked, he was prevented before Joseph could be fully overcome.