Usually, if you are new to it... it is because you're scared and don't feel in control. You have to release that feeling and know it'll end and you did it for a reason and to let it show you what you need to see.

If you're not new to it, it means you're subconsciously..or consciously... repressing something and it is coming out of the dark so you can deal with it. If you feel the devil or some evil force around you, it is not real, it is a part of yourself you are refusing to deal with. Gotta integrate the shadow to become whole.

Or it is just a bad set and setting. If your parents are upstairs and you decide to trip and they are super anti drug... guess what...you're gonna have a bad time.

Edit. As someone who's had a lot of good and bad trips, while my best trips by far were good ones, the bad ones taught me a lot also and I'm equally grateful for those, even if they were difficult in the moment