If you never have a right to violence, then you never have a right to retaliate against your own victimization. And if the government is systematically attacking you, as had been happening to English Catholics for many years, you then either accept your own destruction by the terrible force of prejudiced law or you take up arms against the oppressor. The context of the event is rarely highlighted.
I wasn't around in the 1600's, but I imagine it to be a bleak time if a group of men thought the most effective way they could devise was to kill off the whole of the government. I imagine this is supported by the fact that the government made it a holiday. Try to over throw us? We'll try you for treason, and name a day after you, so you will be immortalized in your failure. It just says to me that they were scared and trying not to show it. Which means he probably was justified.
Guy Fawkes planned to blow up Parliament to reach his political ends. He was, by any modern standard, a terrorist. The prevention of that attack is worth celebrating; Fawkes himself is not worth deifying in the way modern zeitgeist has done. The Gunpowder Plot was, as the rhyme says, treason. It cannot be justified.
The Gunpowder Plot was a scheme to bring down the House of Lords in England. Guy Fawkes was just one of many involved in the plot. The idea was drafted to try and gain more religious freedom under the king. I don't believe it is ever justified to take or destroy another life. Just like we see today, passionate people will twist their thinking to make anything justified. I find the idea of religious people using violence to force their point highly hypocritical.
As I understand it, Guy Fawkes wanted to murder and depose the king because he was Protestant. I do not believe this was a good reason to commit regicide. Guy Fawkes wanted to advance the Catholic faith and felt he could do so by helping to install Princess Elizabeth as the ruler.