One of the very first things that they teach you during Emergency Medical training is safety. They teach you not to go in until safety is completely assured. If there is any possibility that there is danger then we are not allowed to go in and treat the patient.
The same can be applied to other situations. If you see an illegal activity happening then you should contact the police in one way or another, but you should never ever go into a possibly dangerous situation where you could be hurt. If you see someone illegally hanging out on private property late at night and they are drinking and doing drugs then you should not under any circumstances enter the area. The cops should be contacted immediately. You should not enter the area because you do not know if these people are armed with knives and/or guns or not, and they are at least armed with glass bottoms and could hurt you.
Always be smart and be safe because by jumping into a dangerous situation you could also be bringing your friends into dangerous situations because they want to back you up and make sure nothing happens to you in your time of trying to be a hero. Be the hero by reporting them to the authority who has been trained to deal with these situations, not by getting yourself potentially killed or injured.