A .44 caliber handgun

This is Biggie stating that he will assault the aforementioned lesser rappers without hesitation (“get in that ass” is slang for a harsh beating). The adjective “fast” is used here as a pun on the act of fasting practiced by Muslims.

“Steelo” is a slang word derived from the word “style”

“You should be aware of the kind of man I am. I think it might change your approach to dealing with me”

I’m ruthless enough to make it in the crack game, clever enough to make it in the rap game, and smooth enough to make it in swinger communities

I’m also what you would call a quick learner. Check out that upward career arc!

This is an extensive — and impressive — list of things to be jealous of. No wonder Biggie has so many enemies. Haters gonna hate

You mocked me and my style yet secretly desired to be me. (The metaphysical imagery continues with “cursed”)

This line and the next likely refer to a freestyle battle. Biggie is accusing his opponent of using a preconceived verse while his is made up on the spot

Epic biblical allusion. Biggie’s rhymes are “the word” and should be treated with reverence. Look upon them and discover the truth

You’re still riding a bike. I have more rides (cars) than America’s third-larges amusement park (and the closest major park to New York). You are not my equal.

Pedal —> peddle; Biggie is pushing expensive whips in part because he now deals wholesale, not retail, in the drug game.

Biggie pauses mid-insult when he realizes it would be prudent to find out who hired you to come after him than to continue belittling you

Guns, like most machinery, become warm after extended use. Biggie is claiming that he uses his gun so frequently, it is scorching hot to the touch, much like a tea kettle full of boiling water

This line is particularly incisive as it distills the main theme of the song to one simple image. At is essence, “Kick in the Door” is about the law of cause and effect. If you mess with Biggie Smalls, he will hurt you

Tea kettles are host to perhaps the most famous example of cause and effect — if you heat a teapot, it will shout. This concept was of course immortalized in the canonical children’s song “I’m A Little Teapot”