Why did Shree Rama killed Bali by unfair means(if so).

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Friends I am a great follower of Shree Rama. I believe in Rama more than anyone else. But for a long time I have been thinking about a situation in Ramayana. As you might know when Rama had to kill Bali, he didn't challenge him for a fight. Instead he asked Sugreev to fight Bali and then killed him with an arrow. I am very confused about this. Why a person like Rama did such a thing? Why didn't he challenged Bali for a fight and kill him there?