So I went and tried to go back to the surface in the directions I already went to. I found rats scurrying around and I had no choice but to run to the opposite direction as these things tried to bite me.
At some point I outran them and the air started to smell even worse. I found the cause of the smell and it came from a homeless man that was scratched and bitten by a humanoid kind of creature. I blocked my nose and walked as quietly as possible behind the body just in case that whatever caused this was still around.
I thought I was saved as I saw my guide's silhouette around the corner, but he was also killed. Huge rat footprints led to his body. Something large and rat-like stood on two legs; it was tall and skinny. It was holding the body of the guide and eating it. It also wore a lab coat and dark pants that were tearing apart.
My mind had no idea why that thing existed or why it was down in the sewers, but I had to get out of there as fast as possible. The rat-like figure saw me and dropped the body. This was when I ran as far as I could.
The creature let out a high pitched screech as it ran after me. Miraculously I was able to find an exit at the nearby subway station. I wanted to report this sighting to the police but they would probably think I'm insane, so I'll just keep it to myself and warn others that these sewers are definitely unsafe.