How to make your bad guys more realistic.
The last thing you want is to make your chief NPC protagonist two-dimensional, for your players to turn round and say 'well, that's another power crazy megalomaniac dealt with - who’s next?'
The difficult question is how can the players find out about the past of the bad guy? Unless they knew the nasty before they went bad, which can be a brilliant plot hook, the players don't have many chances to get to know them, what with dodging their blasters and bombs.
PC's turning bad is a great idea, and roleplaying games are brilliant for this. If or when a player character turns to evil, the character sheet can be handed over to the GM and then played as an evil NPC. Isn't that a great idea? The character that the players have been adventuring with and getting to know over the past few sessions is now one of the opposite side, adding a fresh new perspective to the game. Maybe the story will follow the same lines as the Star Wars movie Return Of The Jedi, with the PC's trying to convert the evil NPC back to the good side. Talk about high drama. If the players really take to the story well, then the roleplaying opportunities are enormous. This need not only apply to that particular genre. Lets say a player has gotten tired of playing a particular PC, and wishes to retire them or have them killed in a glorious end battle. Why get rid of the character when they can suit a better purpose as an NPC, an evil one at that? This way, the personality of the character has already been defined by the player who controlled it, and if the referee plays that character with the same traits, but with a little more hint of nastiness, the players will respond in a much more eager manner than if they were up against another NPC nasty.
Alternatively, and easier to pull off, the bad guy can be an old friend of the PC's, an NPC who turned nasty, as simple as that. A good plot twist is to have the NPC act like a really good friend to the characters for many sessions, and then the final twist is to find that the NPC is the chief behind all the problems the PC's have been investigating, and didn't want to directly hurt the PC's because he really did get on with them. Nice twist, huh? Bad guys, or at least the characters on the opposite side, can suddenly take to new depths with this kind of personality input.
Why should the chief protagonist be evil? It's pretty much assumed that any NPC who tries to thwart the ultimate goal of the PC's is a nasty. It need not be that way. The NPC who is trying to stop the PC's from finding the sacred diamonds of Lutz, for example, may be doing so out of a religious or belief-driven motivation. Look at the Brotherhood of the Cruciform Sword in the film Indiana Jones and the Last Crusade. That group attacked the heroes straight away, and you immediately think that they are on the wrong side, but it turns out that all they are trying to do is protect the secret of the Holy Grail. Fair enough. They think they're doing the right thing, and maybe they are, but all it was, was a difference of opinion on what the goal of the story meant to each participant. Maybe you could generate that kind of opposition between the player characters. One could be out for the money, another for the glory, and yet another could be thinking 'maybe we should leave these diamonds of Lutz where they are'. It's quite simple to have that escalate into heavy in-party rivalries, but this should be stopped if it seems to be going too far. The main reason why a group plays together is to co-operate and perform as a team, anyway. Unless you’re paranoid.
The bad guy could even take the form of an animal. Yes, it can be done, and if it goes according to plan it can be quite effective. If it doesn't go as well as expected, then it could turn into a cheap re-hash of the Alien movies.
What am I talking about? Well, lets take Ridley Scott’s movie Alien as an example. The film was about an alien life form, which gets on board a spaceship and proceeds to eat the crew, moving and hunting as a predator and running circles around everyone. The creature killed in a nasty way and had a real dark, evil look to it. When it finally gets vaped, everyone cheers and the evil alien is defeated.
But, it wasn't necessarily evil. It wasn't necessarily malevolent. It did what came naturally to it, it fed and reproduced. That is the basic requirement of any life form (unless your playing a totally whacked-out game), and, if you want to look at it simply, the alien was doing what it did.
But that's not how you would see it if you were on the receiving end. This creature would be doing things that would abhor you, eat things that would make you feel sick, move around and kill in a fashion that would scare the hell out of you. Basically, you would think that what the creature was doing was evil. Evil if it was human, maybe, but we can't judge other life forms by our own standards, be they ones from outer space or ones we share our planet with.
Before I get too heavy, I'll get to the point of all this. The protagonist of a game could be such a creature. You can get a very entertaining game if you keep the players on their toes wondering what to do next. You see, they can't guess what a creature is going to do like they can guess the next move of an intelligent being, and that is what makes the game entertaining. They are continuously looking over their shoulders and watching each other's backs. It need not be done with an alien. You can get the same effect with a large predatory lizard, and an even more effective way is to have a swarm attack the PC's in the form insects or some other lifeform, making defeating them much more difficult. The point is, creatures have no qualms about doing what they do. Where the more intelligent bad guys would observe and calculate, the creature would get stuck in. Simple as that.
Well, I think that about wraps it up. I hope that this little piece has given you a few ideas on how to improve what threats the players will face. Remember- if you make notes about what your head bad guy is like, and stick to those traits, then it will make for a more believable NPC. Not only that, when you come to create a new protagonist, you can see what has come before and create an original one to keep the game fresh. Another key thing to remember- don't be too proud of the bad guy you create. Ultimately, they'll be defeated, and after all, you're not in contention with the players.
Of course, you don't let the players know that.