Is forgiveness a fruit of evil wicked kamma?
Was the forgiveness of King Pasenadi Kosala towards Angulimala a fruit or product of the old evil kamma of Angulimala?

The king's initial fright appears to be a consequence of Angulimala's evil deeds.

The king's forgiveness appears to be a consequence of Angulimala's being tamed.

The king's believing what the Buddha said was -- I'd guess -- perhaps a consequence of the Buddha's earned reputation for speaking truthfully, as well as, his seeing for himself with his own eyes.

Since the suttas say "kamma is intention", did Angulimala "will" the forgiveness from King Pasenadi Kosala; similar to how a sexy lady may intentionally seduce a man?

Are you kidding?

Angulimala's "intention" is perhaps as described by the Buddha:


  Angulimala [is and intends to be]  with his hair &amp; beard shaved off, wearing the ochre robe, having gone forth from the home life into homelessness, refraining from killing living beings, refraining from taking what is not given, refraining from telling lies, living the holy life on one meal a day, virtuous &amp; of fine character


... and ...


  [one who] poses no danger to you


I also tried to find what Angulimala's root intention or original intention was -- why was he like that now?

According to Angulimala -- A Murderer's Road to Sainthood (which you're quoting in the question), here is what that said about and quoted by Angulimala at the moment of his "conversion":


  
    (The Blessed One:)
    
    "Angulimala, I have stopped for ever,
    Foreswearing violence to every living being;
    But you have no restraint towards things that breathe;
    So that is why I have stopped and you have not."
  
  
  When Angulimala heard these words, a second and greater change of heart came over him. He felt as if the current of his suppressed nobler and purer urges had broken through the dam of hardened cruelty that had been built up through habituation in all those last years of his life. Angulimala felt now deeply moved by the appearance and the words of the Buddha.
  
  Angulimala's response and what followed is again tersely told in the Sutta:
  
  
    (Angulimala:)
    
    "Oh, at long last a sage revered by me,
    This monk, has now appeared in the great forest;
    Indeed, I will for sure renounce all evil,
    Hearing your stanzas showing the Dhamma."
  


His intention appears to be motivated by reverence for the sage and renouncing evil.

What you ask may be partially true though -- an advertised benefit of the (ethical) precepts is that they provide safety (and so freedom-from-fear, I presume) to all beings -- to that extent (i.e. it's because of those consequences) an intention to keep the precepts may include that intention to affect the minds of others or to share the benefit (goodwill and so on).

It's probably not supposed to be deceitful or impermanent though, e.g. in the way that "seduction" which you referenced is often portrayed to be.

And I think the effects of kamma aren't simple and singular, they interact:


I suppose there are many ways to describe his motive and intention -- e.g. "he does it because he knows what good and evil are"; or "because he sees the Buddha's good example"; or "for his own peace of mind"; etc.
Similarly the effects of kamma are numerous -- affecting himself, the king, and others.
And each person's actions are affected by multiple causes -- the king's forgiveness being affected by not only Angulimala but also by the Buddha, and the king's own ability.