Hi. I am writing a novelization of X-Men (or tv script, haven't decided yet.) This may be an odd question, but it revolves around the character Magneto who is a (presumably) non-practicing Jew who lost his faith during the Holocaust. The story takes place after the mass slaughter of the mutant nation of Genosha and Magneto, discovering he has nothing else, strives to return to his faith. Realizing all the murders he committed during his crusade as a terrorist he has an emotional breakdown attempting, and failing, to justify his actions. How would a rabbi counsel an individual like this?
Unfortunately, I know nothing about Magneto's character, nor his history (i.e. who did he kill, in what context?) I notice you use the term "terrorist", does he take orders from an authority of sorts or is he a lone wolf?
A rabbi would typically emphasize the principle of "Teshuva", often mistranslated as repentance, but in fact meaning return to G-d. Teshuvah requires confession (to oneself and to G-d, not to another person) of those sins and a firm resolve never to repeat them. Ideally, a person would look to over-compensate in good deeds to counter-balance his bad ones.
In particular, in the case of murder, the perpetrator would seek forgiveness from the victim's family and possibly even from the deceased victim (at his graveside). He may go into "exile", i.e. moving from a familiar society to one where he has to start again from the ground up. He would then seek to help restore/ support/ save lives in a form of compensation for having taken one.
The Teshuvah process would include daily reflection and a constant sense of having to compensate for a deed that cannot be undone. This process and these feelings would accompany the person for life.
He would also be encouraged to engage in as much charitable activity as possible, both financially and in person. He would be encouraged to study Torah deeply and consistently and would be advised to focus on daily prayer and the recital of Psalms.
Lastly, the rabbi would advise the person that true reconciliation is about his relationship with G-d and may only be achieved when he himself passes away (although very clearly discouraging him from attempting to take his own life or even neglecting to look after his own health).
Of course, this all assumes that the killings he did were immoral. If someone killed for good reason, like to save someone under threat or even in the line of duty, he would not be considered a sinner, but may still be advised to add in prayer, study and charity to cleanse himself, because killing is never a positive thing.
I hope that helps.
Good luck with your writing!