It is very difficult to see Charles I of England in either of these ways. I would argue, instead, that he should be seen as a person who was simply not capable of coping with the very difficult situation he was placed in.
If we see Charles as a traitor, it means that he betrayed his country. There is no reason to see him in this way. Charles was trying to get power for the monarchy and away from Parliament. This was not an attempt to weaken England or to help a foreign enemy against it. It may have been a mistake, but it was not meant as a betrayal of the country.
If we see Charles as a martyr, it means that he died for a worthy cause. I would argue that he did die for a cause, but it was not a worthy one. Charles died because he wanted the monarchy to be more powerful. He wanted to limit the amount of democracy in England. This does not seem like a worthy cause to me.
Instead, I would argue that Charles was simply not up to the demands of the time. He was placed in a tough situation, trying to cope with issues of foreign policy and religious conflict in addition to the basic conflict between Parliament and the crown.