"When I'm sad, I stop being sad and be awesome instead"

In "How I met your mother" TV series, there is a character Barney Stinson, who is the author of this semi-popular quote:

When I'm sad, I stop being sad and be awesome instead. [sic!]

Obviously, this is grammatically incorrect, because infinite form of the verb can't be used this way. However, I would like to understand why is it phrased this way. After googling a little, I found a few places where people were saying that it is slang.

One of the best (in my opinion) comments also mentioned that

What he meant here was "I stop being sad and choose to be awesome instead." If he said "I become awesome" it would change the meaning. He does not mean that he stops being sad and begins to work on growing more awesome, but that he is awesome.

which, based on the character, seems very plausible. Now, what I really want to know is:

• is this really some kind of slang, or is it just something the scriptwriters invented?
• how would a native speaker understand the meaning of the phrase without any context?

Consider:

I am awesome.

This is a general statement of my state, claiming that it is awesome.

I start being awesome.

This is a claim that I move into the state of being awesome; I was not awesome before, and then I am. It also suggests a degree of agency (I am active in being awesome).

I am being awesome.

This is a rephrase of the first using a doubling of to be to add an emphasis on my agency in being awesome. It's rarely sensible, often awkward, and once very controversial even in more complex forms ("You will be glad to hear, under my own hand (though Rice says we are like Sauntering Jack and Idle Joe), how diligent I have been, and am being." - Letter from John Keats to John Hamilton Reynolds, July, 1819).

Here Keats at least has the excuse that he wants to combine both "have been" and "am being", though some would still say he should have said "...how diligent I was, and am".

Now consider:

When I'm sad, I stop being sad and start being awesome instead.

Standard English, and reflecting a degree of agency, along it being a change of state.

When I'm sad, I stop being sad and am awesome instead.

A change of state is implied by the rest of the phrase, but not by "am awesome" itself. There's nothing to hint at a degree of agency other than it being claimed as a general policy toward sadness. There's a clash between "stop being" and "am", bordering on syllepsis.

When I'm sad, I stop being sad and be awesome instead.

A stronger emphasis on agency, and combines both a claim to always be awesome and a claim to actively be awesome in response to sadness. Be is clearly not used normally, but favouring it over am ties it to the earlier being in a use that again is close to syllepsis.

It's certainly not playing by the rules, but that fits the character, and it's all the more effective for that.

It's not grammatical to say I be awesome but be awesome is allowed in sentences that aren't in simple present tense. Consider:

I'm going to be awesome tomorrow

or

I will go to the party and be awesome

or

I don't have to be sad, I can be awesome instead

Sure, these have "going to", "will", and "can" kicking around, but those are markers of the not-present-tense state of the sentence. And in Barney's sentence, the "whenever I" construct is doing the same thing. He could have said:

When I'm sad, I stop being sad and sing instead.

And if he did, that sing would not at all be the same as

I sing

rather closer to

I sing in the mornings

English doesn't put as many suffixes and similar markers on words to help you notice they are in difference tenses, but they are nonetheless, and "be awesome" is allowed here even though it's not allowed in simple present tense.

It is slang, because the show is a sitcom (situation comedy) where many phrases used are not gramatically correct, but it's fine for us native speakers because we (mostly) understand.