Killing Spacemen is the most important thing. Kill people as soon as you can and as often as you can. The longer the match goes on, the more organized people get, and the more powerful their weapons are. Killing people immediately as the round starts will lower the chances of completing objectives, make the monster level up faster, and generally increase how helpless spacemen are later on. If you spawn in a room with 1 other person, there's no reason to not kill them immediately.

Even bad spacemen can overcome a monster with time and numbers.

Good spacemen will have a very hard time beating the monster with no numbers or time.

Don't waste your time. False accusations and mis-killings will happen by themselves. All you need to focus on is killing spacemen!