Monsters have always populated the fantasy world, but rare is the tale that tells of the monster protagonists. For the most part, monsters have been the opponents and challenges that adventurers measure themselves against, and the archetypal monster, no matter how far the game has evolved, waits within its lair for heroes with hopes of slaying them and feasting upon their bones. Obviously, the appearance of one or more monster characters is going to somewhat alter your campaign. Just how your campaign adapts depends partially on how prevalent monster characters are and what the people of the world think of them. It also depends on the sort of monster characters that populate your adventuring crew.
As a GM, one must decide how NPCs will react to monster characters. Most minotaurs cannot walk into a typical village to buy supplies or get healing. This is not an alignment issue, but rather an issue of expectations. Players now have characters that the world expects to be dangerous, if not outright evil. Initial NPC attitude should be unfriendly, if not hostile, toward monster characters and those who associate with them. After gaining a few levels, a monster character may be a local hero instead, and that reputation may spread to neighboring places.
In the traditional roleplaying game, monsters exist to oppose player characters. Bestiary Levels skew that relationship. As a result, players may find themselves with characters of alignments other than what is normally expected. Some monsters always have a specified alignment, some usually have a certain alignment, and some often have a certain alignment. When a kind of creature always has a particular alignment, individuals who differ are either unique or one in a million. When a creature usually has a particular alignment, individuals who differ are in the minority (considerably less than 50% of the population). When a creature often has a particular alignment, exceptions are common. Make sure your players know why their characters differ from other monsters of their kind, if they do. The more common differences are, the easier it is to justify the change. Remember that the more rare alignment differences are among members of a race or kind of creature, the less player characters expect to find variations. A carefully built and developed NPC may not get a chance to explain if player characters assume all of that NPC's kind are evil. Alignment may well become a particularly sensitive issue among players. Lawful evil people can theoretically be trusted to some extent to keep their word, though they may bend it a little. Chaotic evil characters, however, may need to be watched. On his own, out spying or assassinating someone, a chaotic evil character may be matchless in ability. Other party members might have reservations, though, about letting him pull guard duty alone a night after a big treasure haul has been made. This is not to say that chaotic evils should be banned from play; they just act more independently.
Evil is not stupid. Evil creatures and characters can work together just as well as good characters can. It should be no more difficult to maintain party cohesion with a group that includes lawful evil, neutral evil, and chaotic evil characters than a typical adventurer mix of lawful good, neutral good, and chaotic good. Certainly, evil characters attempt to manipulate events to their personal advantage—a phenomenon not limited to evil parties—but not to the extent of sabotaging their own chances of survival. Everyone needs to be aware that unless some kind of trust can be formed among the players, the game quickly falls apart. Who wants to get involved if you keep getting stabbed in the back all the time? Players with a history of having their characters kill other player characters should know that their characters may become prone to accidental falls from great altitudes, food poisoning, being mistaken for a hunting animal by one's own party, and so on. After repeated incidents of this kind, one may not be invited to any more game sessions, either. Chaotic evil should be played with care. Player characters counting on their fellow adventurers in a crunch, regardless of alignment, have to treat them with respect. Evil characters are still people. Even bad guys have feelings, emotions, and loyalties. This means it is just as possible to play a well-rounded character who happens to be evil as one who happens to be neutral or good. An evil character or creature can be a loving parent (such as Grendel's mother), a faithful spouse, a loyal friend, or a devoted servant without diminishing their villainy in any way; this merely reflects the way in which people compartmentalize their lives and the fact that they behave in different ways toward different groups, brutalizing those they consider beneath them but treating their peers and loved ones with respect and affection. Even monsters that lack intelligence or emotion (such as constructs and some undead) prefer to work with those who aid them or coexist favorably with them. To put a more selfish spin on it, benefiting by the efforts of others is something all but the most psychotic of evil characters should appreciate.
Evil is often more active than good. Good characters react to evil plots, invasions, and crimes; or infiltrate lairs and citadels. The GM decides what the bad guys do, and discover and foil, which is the heart of the adventure. When player characters become evil, players become the driving force for the events of the campaign. They may attack viiages, plunder castlesm raid caravans, or try to open a portal for ancient, elder gods to enter the world.