This is a really interesting question. I have always viewed the poem as a very positive one, a father imparting words of wisdom to his son. I never looked at it that there is a bitterness present in how the father must advise against all of the potential for evil and temptation in the world. This would be why he has to advise him in the way that he does, for so many others have failed to heed such words of wisdom. If this is true, then there is a great deal of bitterness present because so many others have demonstrated the opposite of the words the father speaks. Making the assumption that this is true and that the speaker and Kipling are one in the same, the bitterness present is that there have been so many opportunities for young boys to prove they are worthy of being "men," but have failed in the process. The father's advice is necessitated, then, because of fear that his own son will wind up like others. Perhaps, another layer of bitterness could be extracted if we make the argument that the father was one of these failures, meaning that he speaks from the voice of experience, the one that could not live up to the same ideals he wishes for his son. In a desperate attempt to not relive his own sins on his son, the poem emerges.
I think that you can say Kipling has a bitter vision of the world because of the kinds of things that he is warning "his son" about. If you look at everything he says to his son, he is painting a picture of a pretty negative world. He is telling his son about all the bad things he will have to deal with.
For example, he tells his son that everything will fall apart and go wrong. Not only that, but when it does, people will blame him (the son) for it. He tells the son that he will work hard to build things up and then other, lesser, people will come and tear them down. He, the son, will have to deal with this sort of thing.
This seems to me to show that Kipling had a bitter outlook -- that he thought that things tend to go wrong and that people tend to act badly.
The poem is full of Kipling's bitter vision of the world. The number of of obstacles that the speaker suggests his son will have to face to become 'a Man' attests to the poet's harsh vision of human nature and destiny. The poet advises his son to meet the challenges put by his hostile world with courage if he is to live with dignity.
-In keeping with his bitter vision of the world, in the first stanza of the poem the poet talks about betrayals and attacks. He calls on his son to be patient, remain calm and ignore those who will blame him for misfortunes, doubt him, tell lies about his abilities , and hate him.
-In second stanza he warns of the dangers of losing control of oneself to dreams or being affected by 'Triumph and Disaster'. He calls both triumph and disaster as imposters as both are deceptive.
-In the third stanza, Kipling extolls the idiolistic hero's battle with destiny rather than with others.
-Kipling shows his deep understanding of human nature in the last stanza. It talks about equality and asks to behave alike with commoners and the kings.
-The poet has mentioned twenty six obstacles by striving to became a perfect man. The reception of the word "if" suggests that it is a difficult task.