passover celebrates the night when G-d delievered His people from under the cruel struggle and slavery imposed by the Egyptian captor.
The passover and killing of the first born was the last in a series of plagues visited on Egypt in an effort to encourage the Pharoah to let Israel go, needless to say he did not and hardened his heart to the point where God fixed his choice for him.
Each plague was directed at the prominent egyptian deities. The firstborn was the ultimate deity in the end when it was Pharoah's son.
Imagine the scene... G-d tells Israel to take a lamb (sheep or goat whichever you could get) and kill it, smearing its blood around the doorframe with a hyssop branch. Over the coming days hundreds of thousands of Israelites were slaughtering sheep. The streets must have been running with blood. Not to mention the egyptians that did the same so as not to suffer.
I remember an account that the number of family may have been as high as 100,000 (i suppose that would pretty much use up the entire lamb stock of the country... what a wallop on the national reserves).
Either way, the people did as they were told, and that night the Angel of Death passed over the houses. Those with blood on the door, were OK those without paid the price. We always think of it as firstborn children (in the sense only children died), but it says firstborn, whether they be babies, children, teenagers, adults and animals, all firstborn died.
Pharoah suffering the loss of his son, was gutted commanding the Israelites to leave, now we have 100,000 families plus animals/livestock and their goods and belongings AND any egyptians that chose to leave with them.
Next up came the sea. You know the charlton heston scene were the river splits.
Pharoah had come to his senses and set the army after them to bring them back (after all, the country needed them for construction workers - can't have the common egyptian doing that can we).
Coming to the water, Moses strikes the water and the water moves and the people cross over on dry ground... by the time Israel are across (which must have took a while), Pharoah and his army have almost caught up, but Israel are across and the horses and chariots and riders are drowned when the waters close again.
G-d has delievered His people from a mighty hand...
so why do we celebrate passover.... because G-d delivers when no one else can. blessed be His holy Name. We do it to remember, our captivity, our slavery, our deliverance.