who is responsible for Gatsby's death and why?

Daisy. Since she is the one who defines Gatsby. Oh, and she was driving ofcourse.

Mr. Wilson, Myrtle husband is the one who kills Gatsby. Mr. Wilson thinks that Gatsby is the one that his wife is having an affair with and that is why he ran over her which it is not true. Tom fail to tell Mr. Wilson that his wife Daisy was driving Gatsby car and was the who hit Myrtle and he also fail to tell Mr. Wilson that he was the one that was having an affair with his wife not Gatsby.

I believe FSF would say that Gatsby was killed by the entrenched society represented by the inhabitants of East Egg. The notion that access to the class, panache, cache and general swellness of upper society can by purchased must be eradicated. The real American Dream, which Gatsby represents, has been identified and compromised; attempts to bridge into upper crust society must be "exterminated, exterminated with great prejudice".

i think it was mainly Daisys fault.She did not admit to killing myrtle although she did know. Mr Wilson is half to blame though as he pulled the trigger. If Daisy admitted to killing Myrtle, Gatsby would still be alive.