How do I write an Insane Person?

For the story I'm currently writing, I plan on incorporating a person (bad guy, obviously) who is mentally insane. I would even say borderline psychopath. I really think this would make the story feel complete, but I have absolutely no clue how to do it. I don't want to spoiler the story too much, so I'll try to be as vague with the information about him as I can.

-He is high up on the chain of command in his organization. Not the boss, but close to it.

-He is willing to do horrible things like kidnapping, brutal interrogation (not torture, I'm not ready for writing torture), robberies, etc. for his organization to succeed.

-How can I write his speech? It would probably be different than a normal, sane character, right?

-His main goal is to see his organization succeed. NOTE: The organization is about making money.

Once I think of more details, I can post them here.

Gotta love insane characters. Frees you up to write all sorts of fun stuff! >:D

And that's the key thing to remember: there are a lot of varieties of insanity, so that alone can't be the defining characteristic of your character. It's good that you've already settled on a motivation and some restrictions on your character's insanity, i.e. they can't be so far gone that they're unable to hold a position in an organization.

As for how to handle his speech, that's a very good question. I have a follow-up question that I think will help you find an answer. Is his insanity supposed to be something that's apparent from the first time we meet him, or is it supposed to come as a surprise later? If it's the latter, then I would suggest making him seem as normal as possible, but leaving enough hints that something's off that when it's revealed the reader is surprised but makes connections with what's come before. Maybe he jokes about doing terrible stuff and everyone thinks he's a clown, and later the main character finds him actually doing it and joking about it in the same tone as before while he's doing it.

Basically, the difference between his mode of speech and a sane person's should be proportional to how obvious his insanity should be to the reader and to the characters around him. If the other folks in the organization all know he's totally nuts, you can reinforce this by: having him trail off frequently in conversation, having him go off on violent tangents, having him get really emotional for seemingly arbitrary (or maybe actually arbitrary) reasons, etc.

One last bit of advice I'd give is this: don't confuse insanity with irrationality or being excessively emotional. Although an insane person certainly can be irrational and emotional if that's how you want to write him, an insane person can also be rational to the exclusion of everything else. A person who thinks perfectly clearly and logically but is utterly devoid of empathy and has no negative reaction to others' pain or his own pain would be called insane as well. If that's not your cup of tea, maybe he has empathy and thinks logically but starts from false premises brought on by hallucinations, or dreams that he is unable to distinguish from reality.

Lot of different directions you can go with this. Hope this helps, and best of luck!

^i feel like iconmeanon's secretly insane lol

I did have an insane character at one point that bordered between psychopathy and insanity. What I can say is that there are different types of psychoses: you have that Joker kind, always laughing and playing tricks, and the Ra's al Ghul type, dark as hell and murderous as hell (sorry if you don't watch Batman that's literally all I can think of right now) and probably a lot more in-between. One common thread is that they don't perceive things the way a normal person would, be that with relationships, visually (hallucinations) or otherwise. So I think that's something you should have with that character whether you want to make it really obvious or not.

If you want to write a psychopath, the important thing to remember is that generally you can't immediately tell that there's something wrong. For all intents and purposes they appear as ordinary people until they start doing stuff like killing people because they have no ability to place a value on life etc.

Of course, there's a lot of different disorders etc that can be called "insanity" so I'd suggest doing research into them.