To answer this question, it is important to consider three different points of view - that which agrees, that which disagrees and that which offers an alternative perspective. It would be ideal to offer at least two examples for each of these sections to achieve a balance in the answer. For example, we could say that Charles' intransigence was a key cause of regicide based on a) his handling of the Newcastle Propositions, and b) his actions towards the Scots. It is important to be specific when answering a question like this and always explain what you mean. It is better to be clear than to try and sound clever as this can mean you might becoming overly complicated. It would also be a good idea to include some historiography - the argument of a published historian. But, if you decide to do this, you will need to respond to this argument, and suggest how it either compliments what you are saying or disagrees with it. The most important part of answering this question is knowing what you think yourself. When you are sure of your own argument, the power of your writing improves massively and you will sound all the more convincing.