If I quit my horrible job and not inform universal credit about it, could I get into trouble?

FRAUD. You have to inform them or your money will be stopped.

Just so Americans here are not confused: Universal Credit does not exist in the US.

you will see. If you anticipate finding it hard to make payments on time or need to stretch out the contract, call them and ask to have the deal worked out. If you've been a good customer, they probably will work something out. They just want their money. Do not wait until you run out of money to ask for mercy. Maybe you can get another job before this goes critical.

call them and ask what would happen if i quit my job

most likely yes

You have to tell the organisation about any change in your circumstances, no matter whether it happened to you, or whether you made it happen.

nope -
but since your universal credit is reduced cos you have a job , it aint gonna increase when you leave just cos you dont like it

that is - all that leaving the job will do is reduce your income by how much the job pays

If you tell them you have left then UC may eventually increase

OK if you keep paying any payment due, on time