How does Victor Frankenstein's lack of personal responsibility lead to tragedy? I have chosen the decisions of making the monster, not clearing...

The first two supporting events--Victor's decision to make the monster and not clearing Justine's name--work well for Victor's lack of responsibility.  A third example, that might be better than not making the female monster, is Victor's seeming disregard for Elizabeth's safety until it is too late.  Here are some suggestions for each of your points.

1. Victor's decision to make the monster and then doing nothing to take care of him demonstrates his selfishness and unwillingness to accept that what he does has consequences not just for him but also for many others.  He creates the monster to bring himself glory, and during the process thinks nothing of those souls' whose corpses he descecrates or what he should do to help this new "human being" that he will unleash on the world.

2. When Victor realizes that his creation has brought destruction to his family and town through the murder of William and false arrest of Justine, he cowardly comforts Justine but is unwilling to take responsibility publicly for William's murder (by telling about the monster).  He thinks that others will think he is crazy; so he selfishly allows Justine to be executed.

3. While Victor's decision to destroy the female monster might be seen as a lack of personal responsibility, one could also argue that he has finally taken responsibility because he does not want to unleash another monster upon the world.  So, if you focus on Victor's misinterpreting the monster's warning about being with Victor on his wedding night, you will have more proof for your thesis.  Victor thinks so much of himself that when the monster makes his "wedding night" threat, Victor assumes that the monster will kill him.  He never even considers that the threat is toward Elizabeth.  He carelessly marries Elizabeth without telling her or her father to be on guard and what to look for, and he leaves her alone on their wedding night.

In regards to starting your paper, if you have to write a normal five paragraph essay, a good introduction for yours could discuss personal and corporate responsibility in general and narrow down to your thesis about Victor.  Victor's "product" (the monster) is similar to defective items produced by companies which cause injury or death.  You could also focus on the lack of personal responsibility in the world today and how that leads to problems for others.

Good luck--it sounds like you have chosen your topic and supports well!