1- Yes the definition of La’na is the withdrawal of mercy. It basically means we ask Allah to punish the evildoer for his evil.
2- Imam Ali (a) fought the evil doers and killed them. During the Prophet’s (s) life he killed many pagans. At Jamal, Siffin and Nahrawan he killed many. Sometimes evil doers must be fought because they want to spread their evil to others and endanger them. Justice means that they must be fought. They cannot always be forgiven in the name of mercy, because they’ll continue their aggression.
3- So then why did Imam Ali (a) show mercy to the one who struck him? Some possible reasons:
One: some people may have thought that Imam Ali had a personal problem with Ibn Muljam, and so they would have relegated his position from fighting to the truth to just having a person problem. Imam Ali did not want to give that impression. The way to do that is to treat his striker humanely.
Two: some people wanted to seek revenge, attack many people to avenge the death of Iman Ali, and cut Ibn Moljam to pieces. Imam Ali wanted to stop that, so he treated him this way. In his will to his son he said “make sure he isn’t dismembered and disfigured, for the Prophet banned that.”
Three: sometimes you want to show how vicious your enemy is. If Imam Ali had dealt with him roughly, some people may have felt bad for him, and the Imam didn’t want that. By treating him good, Imam Ali further showed just how evil his enemies were.