Both endings are good in their own way. Revenge ending makes sense after what Dimitri put him through, Deal ending makes sense from a character point of view. One can interpret it as a choice influenced by continuous persuasion of Roman, or as Niko's own deviation from morals and principles. After the trauma of war, the betrayal of Darko Brevic and death of the twelve men from Fifteen Man Squad, he came to Liberty City but realized he is living no better life. Growing up in such an environment, he decided to compromise his morals and paid heavily for it.
