Antagonists are those individuals who are against what player characters are doing. They might be evil wizards in their towers or town sheriff who make things different for adventuring party. Antagonist is not always monster or bad guy. Antagonist might be someone -or something- that has different goals to what characters' goals are.
Antagonists should have some features to make them important and feel like real characters. You could make these features stick out as you want antagonists to stick out from normal enemies.
Antagonists don't live in the game world just to be against characters. They have their own motives why they do what they are doing. Character's are just crossing their path and conflict follows. Many fictional antagonists do vile things just because they are evil. If there is evil wizard who tries to put magical crystals together to destroy the world it it can be a great adventure for players. But why does that evil wizard want to destroy the world? Is he just insane or does he think he has greater insight? You should determine why antagonist is doing what he is doing and what would be his award if he succeeds. Not necessarily player characters will ever find out his motive but as a GM you could possibly play the part of this antagonist better knowing his thoughts. So write a motive. It can only be few words as it is better than nothing.
How Far Would He Go
Not all antagonists are ready to die for what they believe. But in several games that's the case. Characters thrash antagonist's plan and continue to kill him. As a last straw when everything is already lost for antagonist he will fight to the death. But why would he do that? Isn't it enough to loose everything? All the captured guardian monsters, lots of treasures, possibly that doomday's machine is trashed and so on. He has nothing. Of course as there's nothing left his life won't matter and it could be logical to try to at least kill the guys who destroyed your plan. But why cannot antagonist be a whimp? He had these traps and guards and what-not to protect himself because he is not a fighter. Wouldn't it be worse punishment than death to face jail? Plus, he could also always later make a comeback... with a revenge. Or he might even turn his sledge and help player characters later if he did see wrong in his actions and changed his ways.
LotFP has morality. Enough damage dealt roll d12 against morality. Roll higher and opponent flees or something. You could easily assign morality for antagonist. Lower score for cowardly and as high as possible for insane megalomaniacs. That way antagonist won't fight to his death but can try to flee or even surrender when his hitpoints trigger it.
To make antagonists really different they might posses one or few special abilities not included in game rules. That makes them totally different. But is it fare for players? Why not. Antagonist has to be somehow special because he has gone that far already. As player characters usually are special from normal people in game world. Player characters have spells, feats, better equipment and other gualities others don't possess so to make antagonist really the bad guy he should also have something. In low fantasy game antagonist might have robotic hand. Why not? He is one of the kind and might possess one of the kind technology or knowledge. Or he might have some special skills or powers what others don't have. It also surprises players - also those who know the rules like their own pocket.
There could be a weak point. Daughter antagonist won't put in danger or some valuables he doesn't want to loose. Basically this weak point could be alternative solution to win antagonist. But it should be hard to find out. This alternative method to beat antagonist through his weakness could be so well preserved characters never find out about but it should be there for alternative resolution. Some antagonist might have obvious weakness and others hard to find out.
You don't have to write a short story of the antagonists past but you should know why he does what ever he does. Was he bullied as a kid and now wants to be powerful and pay back? Is he just greedy or does he want power no one can imagine? This antagonist player characters try to stop destroying the world should have reasons in his history why he does want to destroy the world. History is tied really close to motive but only deepens the antagonist.
If antagonist has castle with valuable treasures, monsters and traps guarding it and other things you should think how he obtained them. Also if there is that death trap castle filled with monsters how did antagonist build it and how does he manage it? It should have other meanings than just being a challenge for player characters. Obviously antagonist didn't put it all together just for wait adventuring party to step in.
Make Antagonist A Character
Basically in game mechanics antagonists are part of an adventure and challenge for player characters but to make antagonist more alive you should think about these things in this post. Antagonist would be more interesting person if there is something else behind him than just statistics to fight against.