Q: Could a person with schizophrenia have such a mild version that, to most, he appears completely normal except for a few "quirks" and his bad symptoms only come out in periods of high stress? Or would I be speaking of a completely different illness?
The villain has murdered a man because that man gave him bad investment advice and our villain lost everything - money, family, house, job, etc. Then, when the heroine comes close to outting him, he threatens to murder her too, because she poses to destroy everything he's built since the first murder.
A:No to the first question and yes to the second.
Details on other disorders you could use (some of which are on the "schizophrenic spectrum") in a sec. There's one I think is an almost perfect if not perfect fit; I go through other possibilities first so you can see how they fit and don't fit, and give you that one that sounds best based on what you've told me at the end.
First let's look at why schizophrenia doesn't really work -- and by the way, most movies and books default to schizophrenia without realizing these things below, which is why half the killers in fiction supposedly have it and yet have no symptoms of it! So it makes total sense that would sound like the right diagnosis if you didn't know the clinical info. Thumbs up for asking!
Schizophrenia is, almost by definition, one of the most crippling mental illnesses you can have, if not the most crippling. Because it has such a profound effect on one's quality of living, it's going to be fairly obvious to anyone who spends any amount of time around the person that something is up. In fact, the majority of people with schizophrenia aren't able to live completely independently.
While some people who have schizophrenia have periods that are much better than others and may mostly seem odd (think very eccentric more than a little quirky) during those periods, when the symptoms flare up, they're bad, because they include positive symptoms and negative symptoms (you don't usually see both at the same time).
Negative symptoms look kind of like a super-extreme depression and include
Okay, so if schizophrenia won't work, what might? Here are a few options.
These are schizophrenic spectrum disorders, which means researchers think they may be milder forms of schizophrenia; that is, you got some of the genes but not most of them:
This is different from schizophrenia because it's much milder and the person is kind of odd but not to the point that they can't live independently. These people do not have hallucinations or delusions -- as soon as they have either they're into full-blown psychosis and get a different diagnosis. There's a lot of inaccurate info on the web about this (people accidentally refer to schizophrenia symptoms as schizotypal symptoms) but I found a good reference for you on the Mayo Clinic website. 
I don't think this is where you're going, but if you look at schizotypal, you're going to see references to schizoid, so I figured I'd better reference it. This is the only condition I know of in which you just don't want to have relationships with people, of any kind. You see them as messy and undesirable...to the point that these people don't have friends, don't want a family, don't even want to have sex with others because that's too intimate. They just sort of keep to themselves; you want someone who's going to interact.
Neither of these types of people is likely to be violent.
Convinced that people are out to get him, enormous chip on his shoulder, but never really acts "normal," is like this all the time. This kind of person might be more likely to be violent to defend himself, but I'll be honest with you, personality disorders are hard to write because in my experience the people always feel a little "weird" to the average person.
So...one possibility:
This is one personality disorder that's not too hard to write about, once you get the symptoms down. People who kill and don't feel bad about it, ie they don't have a conscience have APD. By no means do all of them kill; they may also be con artists or just normal people who don't experience guilt but choose to live by the norms of society because it's easier. What makes these people seem less "weird" is that if they're smart, they can sometimes cover the disorder by being super charming. But -- those whom I've met with it haven't had any problems telling me (and others) that they don't feel bad about things...they will actually boast about it, and crimes they've committed. If they're suspected of a crime, you're better off telling them you don't think they could pull it off so they'll correct you than try to make them feel bad or guilty. There is no guilt. So he'd stand out as more pathological than quirky if you could get him talking. He'd know he wasn't supposed to kill people, but he'd do it anyways. The wikipedia entry on this is decent.
The disorder I think is the best fit based on what you've told me:
This is someone who can seem totally normal except for one area where they have weird ideas that aren't based in reality. They can be things that are completely believable (I'm friends with a celebrity, the FBI is after me, someone wants to bury me alive) to bizarre (aliens abduct me every week, all of my family members have been replaced by robots that look just like them, all birds are spies for the government). Nonbizarre tend to be more common, to my understanding, for this...you start to lean toward schizophrenia with the bizarre. There are different categories of delusional disorders, including paranoid, and they do end in murder sometimes. This sounds just like what you're talking about, especially because high stress will probably make it worse, especially if there's paranoia. If this person killed, it would feel totally justified to the person, just like what you're talking about. There's a great overview at eMedicine.
