There are two types of suspects in murder mysteries: sympathetic and unsympathetic suspects. Oh, a suspect can go from one category to the other. We might not know which category a suspect is in. But ultimately, every suspect will be sympathetic or else not. So, which is better?
I can already all but hear people clamoring that a mystery story needs both. These people are probably right, but “both” is no fun as an answer so bear with me while I consider the question as a simple binary.
The first thing to say in favor of sympathetic suspects is that they are more pleasant. One can stand to read far more about sympathetic characters, in general, than unsympathetic ones. This is an important consideration. There is a danger here, though, which is that if a sympathetic character turns out to be the murderer, it is something of a blow. This is often a popular way to disguise a murderer, but it runs the risk, when overdone—as it often has been—of unintentionally conveying the idea that apparently good people always have some secret evil which they are hiding.
I should clarify that a sympathetic character need not be an outwardly moral character. It is possible to make an explicitly immoral character sympathetic, though care should be taken here not to let that lead the reader to approve of the sympathetic character’s vice. I will say that this sort of sympathetic character would be the better choice for murderer, if one must make the murderer sympathetic.
That is especially true in the modern era, where the general trend is in favor of excusing all vices, or at least almost all vices. Tempting people to excuse the vice of the sympathetic immoral person and then it turning out that they were merely being taken advantage of—there may be a useful corrective in that to the general vices of our time.
Probably the strongest argument to be made in favor of unsympathetic suspects is that it is often better for the murderer to be an unsympathetic character. Having said that, this does have the unfortunate consequence that one must have several unsympathetic characters in order to avoid making it obvious who the murderer is. (I’m speaking in generalities, of course. Enough gambits have been tried, and bluffed, and double-bluffed that one can probably pull off making the murderer unsympathetic, and making everyone suspect him, and starting off with some pretty damning evidence against him, only for the reader—and detective—to refuse to believe it because it’s too obvious. I can’t think off-hand of an Agatha Christie in which this was done, but if anyone could pull it off, she could.)
The downside to unsympathetic suspects is so obvious it may go without saying, but just in case it doesn’t: they tend to be unpleasant to read about. One must, therefore, keep their “screen time” short. A little bit of them will, usually, go a long way. This puts us in an especially bad place if the murderer is unsympathetic, since then we’ll need to have at least a few unsympathetic characters. Several characters, all of whom get fairly little time on the stage in order to spare the reader, will be harder to keep separate in the reader’s mind.
It will be well, therefore, to make them more stylized, in order to make them more memorable. One way of doing this is to give them vices which tend to be excused in popular culture. Poking fun at the vice will be more tolerated in an unsympathetic character, and it will be more memorable. It will also probably be more realistic, too, since vices have a tendency to cluster (as several vices often share a root cause).
In the end, I think that there’s a lot to be said for having a good number of sympathetic characters. Care must be taken if one of them is the murderer, but care will always have to be taken in writing a murder mystery.