Which character in The Great Gatsby could you argue kills the main three characters?

It is not really a single character who kills Myrtle, George, and the title character, Jay Gatsby. It is instead a certain trait: carelessness. If you have to blame a person or two for causing these deaths, you could argue that Tom and Daisy Buchanan embody carelessness. However, they are not the only characters who express it.

Earlier in the novel, Tom treats Myrtle abominably during his visit to the city, even resorting to physical abuse. However, Myrtle also treats her husband, George, abominably. She inflicts Tom's cruelty and indifference toward her onto George -- a man so inept yet vulnerable that one cannot help but to pity him.  

Nick Carraway, the novel's narrator, sees Tom again after Gatsby's death, "walking ahead...along Fifth Avenue in his alert, aggressive way, his hands out a little from his body as if to fight off interference..." (178). This description of Tom's physical presence reinforces the novel's characterization of him as a man who not only believes he can control everything in his environment, but that he has a right to do so. 

During their brief encounter, Nick quietly assesses Tom and Daisy as follows:

I couldn't forgive him or like him, but I saw that what he had done was, to him, entirely justified. It was all very careless and confused. They were careless people, Tom and Daisy -- they smashed up things and creatures and then retreated back into their money or their vast carelessness, or whatever it was that kept them together and let other people clean up the mess they had made.... (179).

The final sentence ends with an ellipses to indicate both Carraway's uncertainty and bewilderment over this thought. Tom, to the very end, takes no responsibility for either Gatsby's or Myrtle's deaths. We never know whether or not his wife confesses to him that she was the one behind the wheel of the car that cut Myrtle in half but, it is possible, in the fictional contexts of their lives, to imagine that she would have. Daisy sent neither a message nor a flower to express her condolences for Gatsby's death, behaving as though he had never reappeared in her life and, therefore, did not matter.