To be fair, his dead dad asked him to...

His father's murder, at the hands of his uncle and mom.

He killed his uncle and mom, and his sister committed suicide. You can argue that Ophelia was driven to suicide, but that's not entirely on Hamlet. He might have saved her if he wasn't so consumed with vengeance, but that's not how the story goes.

Or did Hamlet just think his uncle and mom killed his dad because he was a crazy guy who talked to "ghosts."
- It can legitimately be argued either way, which I think is fun. Obviously the play is from the point of view of Hamlet, so we're expected to roll with the assumption that he's really really speaking with ghosts. But it could just as easily be stress induced delusions.

It was the vampires. They should have stayed at their home base.

Yorick was the jester, all the more ironic his "chops" are now fallen and decayed. I think this scene is just reminding that we're all going to end up that way, no matter what we brought to the table when we were alive.

I think a lot of Hamlet can be interpreted about being about the inevitability of death. The famous "to be or not to be" speech is all about whether life is preferable to death.
- I think it's also important not to overindex on universal metaphor.
  
  he, the prince, just found out his father, the king, was killed by his uncle to take both his mother and the crown and he feels he has to face the entire political and royal machine of the country against him - and wonders if it might not be better to just die.
  
  Considering the character is also named after Shakespeare's son who died (Hamnet Shakespeare, with an n) - it could also be a maudlin / macabre reflection on his own role as a father.
- I also like to think that speech is about "to be BOLD in this moment, or not to be," as in, to take action can lead to dire consequences, maybe it's safer to sit back and "not to be" at all. But it also is about his contemplation of death and whether 'tis nobler in some way than life is.