Amateur Thieves

Three muggers ended up beaten up after trying to commit an armed robbery. They had managed to enter a flat and pointed at the residents with their guns. On seeing themselves threatened, the residents handed over all the money they had to the thieves. All was going very well to for the muggers until the victims realized that the guns were fake. As the victims outnumbered the thieves, they began to hit them, first in the flat and then outside. The thieves tried to get away, but they were caught and brutally hitbeaten. When the police arrived, the thieves were badly injured and one of them had to be taken to the hospital.

"ended up beaten up" is grammatically correct (past participle) but "ended up beat up" sounds more natural to me.  Not sure why!

Also at the end, "beaten" seems like a better word choice -- it's more of a process, and it might include kicking, scratching, slapping, whatever, whereas "hit" is a fairly specific word. 

Overall, your writing is very clear and shows great sentence variety and structure.  Fantastic!