Aaron and the priests want a monopoly on sacrificing to God, so any man found killing an animal for the purpose of making an offering to God outside of the Tabernacle is to be killed.
The reason why is stated right in the text:
The reason is so that the sons of Israel may bring their sacrifices which they were sacrificing in the open field, that they may bring them in to theLord, at the doorway of the tent of meeting to the priest, and sacrifice them as sacrifices of peace offerings to the Lord.
You can’t worship God on your own, people, you need a priest to do it for you and consequently collect a fair percentage of your offering for his own use.
God would also like the people to stop worshipping goat demons. Goat demons? Seriously? Why are we just hearing about this? That sounds a lot more interesting than 5 chapters of designing a tent and 5 chapters about building the tent…
God repeats himself and says seriously, if you try and make an offering to me and don’t involve the priests, you should be killed.
God: Psst… hey… Moses…. Mooooooooseeeeees. I know you can hear me, Mooooses… tell the people don’t eat the blood. Never eat the blooooooood. (whispers) the life… is in… the blood, Mooooooses… the life is in blood….
If you eat blood, you die. I guess that precludes ordering a rare steak. That hurts, Lord… that really hurts…
And if you eat roadkill, you have cooties until you do your laundry and bathe and wait until sundown. If you don’t bathe and do your laundry, they should kill you.
God: Hi, Moses… whatcha doing?
Moses: Mostly living in fear, what’s up?
God: Oh, just got a few rules on what you hairless monkeys can do and shouldn’t do with your wee wees and your hoohahs. For starters: No more incest.
Moses: You… wait… I’m okay with this, but considering the book of Genesis was filled with blood relatives having sex, I’m a bit shocked.
God: Yeah, I’ve just been thinking it over and it’s just kinda gross now. So no more. Also, that extends to in-laws, so no sleeping with your brother’s wife.
Moses: Wait, didn’t you kill a guy for not wanting to have sex with his brother’s wife?
God: Hmmm… oh, yeah… yeah I did… okay, no sleeping with your sister-in-law while your brother’s still alive then. Also, no mother-daughter three ways, those are right out. And no marrying two sisters while they’re both still alive.
Moses: Okay, just a minute. That’s exactly what Israel, our founding father did. And now you’re not cool with it?
God: … what’s your point?
Moses: I just thought, well, everyone kind of talks about sexual ethics like they’ve always been written in stone and don’t change, and you’re kind of changing things up here.
God: Things change. Get used to it. Things might change in the future too. Like, remember how I said if you had sex with your menstruating wife you’d both need to wash and would be unclean until evening? Yeah, changed my mind. It’s gross. Stop it. No adultery, we already covered that. Don’t sacrifice your kids to gods, and yes, I know, I told Abraham to do that, but that was a joke. Who knew he’d take it seriously?
Moses: Anything else?
God: No gay stuff.
Moses: Then why did you make gay people?
God: I don’t know… things were so busy then. I had put off this whole creation thing like forever, and then the deadline came on so fast, I had to get everything done in like six days…
God: But hey, I didn’t mention lesbians, so I guess they’re still cool…
God: And this should really go without saying, but don’t screw animals.
Moses: Even if you’re cold, and it’s been a while, and that horse is looking at you like she wants it?
God: Neigh means nay, Moses. Neigh means nay.
God: Anyway, if, uh, anyone has a problem with any of these, go ahead and kill them, m’kay? Just remember… (whispers) I’m always watching, Moses… always…