I like to think Kenny refused to leave another person behind and wanted to defend the kid as long as possile. His cold-heartedness and survivalist mindset just snapped and he couldnt give up on the kid, especially after seeing things from his point of view and feeling sorry for him.
Of course he may have just been suicidal (he has lost his family and boat) and saw it as a good way to go out. But i like the first explanation more
