Is it possible to delete my account and all its activity?
If I want my account to disappear with all its questions, comments...etc. from the stackexchange website is that possible?
No, it is not possible to erase every content you posted on SE. When you created your account, you agreed to this:
You grant Stack Exchange the perpetual and irrevocable right and license to use, copy, cache, publish, display, distribute, modify, create derivative works and store such Subscriber Content and to allow others to do so in any medium now known or hereinafter developed (“Content License”) in order to provide the Services, even if such Subscriber Content has been contributed and subsequently removed by You.
The software itself will prevent you from deleting your questions with upvoted answers, accepted answers, and/or multiple answers. Moderators get (AFAIK) some kind of notification when a user starts deleting a large number of their own questions and answers, and typically revert the actions in question. Your questions and answers with a negative score will be deleted when your account is deleted, though. For more information about all this, see this link.
I guess you could start deleting all your comments; I don't know if moderators are notified of this, so maybe it will get under the radar. I have to say I don't really see the point though.