Book Reports / Who causes hamlets decline and death [2]

I have to write an essay and the topic is: who is the main character that causes the decline and death of hamlet. So far my three main points are 1. Claudius kills king hamlet 2. He has people spy on hamlet and 3. He marries Gertrude. If you guys have stronger points or have any ideas on a creative take I could have on it that would be great. Also if you have any major points that I should include that would be greatly appreciated