The tauroctony is one of the central myths of Mithraism, the story of how Mithras became a god. How much of it is literal event and how much is symbolism is a matter of hot debate; what's certain is that it's not a how-to manual on achieving godhood - more a story of Mithras's glory and why we look up to him. The tauroctony is one of the most well-understood facets of this world's Mithraism as well, as a depiction of it appears in nearly every mithraeum, and yet we understand very little of its symbolism, and I'm afraid I don't have much insight into a lot of it. (Why the scorpion? Why the dog and the snake? I can take guesses, but none confident enough to write here.) Something that does seem to have gotten lost that I'm certain of, however, is why Mithras needed to slay the bull. Modern people rarely think of cows as dangerous, but a full-grown wild or feral bull can be a serious threat. They're massive animals with horns and hooves that can kill, and they can and will attack and kill people out of defensiveness rather than running away, not to mention damaging property and crops. This one in particular is said to have been "running wild," and it may have been injured and enraged, diseased, or even cursed or supernatural in nature. It may have also been a true wild bull, an aurochs, making it even larger, stronger, and more dangerous; such animals didn’t go extinct until the 1600s, well after the time of the Mithras myth’s origins. For that matter, it may have even been an Apis, a wereaurochs, corrupted or otherwise turned violent and destructive. Whatever the details, this is not a story of Mithras hunting an animal down arbitrarily; it's a story of Mithras protecting the people from a very real threat and putting his own life on the line in order to do so. (It may also have been a test or a bet, I’m not sure - something Sol challenged Mithras to do in order to prove himself worthy of the title of sun god, maybe, or something some other god or oracle pointed him toward. It certainly impressed Sol either way, enough to allow Mithras to reach out and claim the power of a god. And, in all fairness, riding a wild or feral bull to exhaustion without even a rope to hang onto is a hell of a feat.)
As the story tells it, the young (and yet-mortal) Mithras, armed with only a knife, hunted the rampaging bull down and vaulted onto its back. He clung to it as it bucked and raged, remaining firm through all attempts to dislodge him, and let the bull's own rage wear it out, instead of trying to overpower it with brute strength as others had presumably done. When the bull had exhausted itself, Mithras took the bull by the nostrils and drove (or in some versions carried) it into a nearby cave. There, he sacrificially slaughtered it with the knife he bore, dedicating it to Sol and calling him down from the heavens. (Whether Sol is an older sun god or a personification of the sun itself depends on who you ask.) They shared the feast together, and Sol knelt to Mithras and shook his hand, passing the power and responsibility of the sun on to him. Once this was done, Mithras ascended to the heavens in the sun-chariot and took his place as the new god of the sun, soon to be followed by many of his other epithets.
In most versions, Mithras becoming a god is the same event as him becoming the first vampire. Some contest this, proposing that he was a vampire first and attained godhood after, or even more unlikely that it was the other way around. To me, it makes the most sense for the two things to be one in the same - his power is passed on through the Blood, and there's no separation between divine and vampiric power there. Every vampire holds a shard of divinity within themself, and that's what sustains us beyond death and gives us power beyond mortals. But that's just my take on it.