Because the king had enforced new laws against the catholic religion. Guy Fawkes was just one of the group of Roman Catholics that were trying to kill the king. Robert Catesby was the leader of the gang and from a very young age he saw his father getting thrown into prison for being a mere Catholic, perhaps that what made Robert the man he was, a man wiling to take the law into his own hands.
Because Guy Fawkes thought that the king was being to harsh to the catholics.
He wanted to kill the king because of the new protestant laws
Because he want to be the King.