The Monster party exists primarily to create fun game for the Players, whether this be through providing appropriate challenge through encounters, playing compelling NPCs, or just generally bringing the Mission to life.
This is the guiding principle on which all Monster behaviour should be based.
That said, Monstering should be fun and enjoyable as well, and the GMs should facilitate this as one of their primary concerns.
Be careful when choosing weapons that the set-up you have will provide an enjoyable fight for both sides. In particular, polearms and shields should be chosen with care, as they are hard to fight against with some weapon combinations.
Unless you have been specifically briefed otherwise, Monsters should never attack or execute a PC who is unconscious.
Monsters should try to make their actions understandable and predictable to the Players so that they can understand what they are doing and how to defend themselves.
If Monsters feel that the Players are having a tough time of things, or if they are not being challenged sufficiently, they should discuss this matter with the GM in charge of the Monster party.
The GM in charge of the Monsters has ultimate call over stats and decisions about Monsters and should be listened to by the Monsters.
Everyone who attends Planetfall is expected to Monster as much as they play (one week Monstering, one week Playing, generally speaking, though this may have to be a higher ratio when we have lots of people who would like to play).