If you destroy the contents of the card it doesn't look good, that fact can be brought up against you (tampering with a car that was involved in an accident especially one with casualties could potentially be a crime on it's own).
If you present only the snippets that support your case the other party will petition the court to for you to present the unedited video if you cannot the entire thing will most likely be thrown out (the other side might be able to present the fact that you have attempted to present an edited video and have discarded the raw footage which will likely not fair well with a jury).
Not to mention that having the video it self could put you at risk of perjury or being found in contempt if you try to do anything after the fact.