I feel like this is another it depends thing. Killer can ranger from self-defense to revenge and such. An abuse victim who kills there abuser is still a killer. But I'd sympathize with the killer because of what they'd been through. A rape victim who kills their rapist is in the same boat; they are still committed murder but I feel like that's self-defense/a pretty damn good reason to kill someone. If that makes sense.
Where as someone like Diane Downs who killed her own kid and tired to kill her other two is far less sympathetic and I'd probably give someone a funny look if they sinabi that they sympathized with her. I also tanong that group of people on tumblr who idolize and romanticize the Columbine shooters.
I guess in the end it's also about how far it's taken. School shooters is a good example; there is a part of me that feels bad for them, but it's madami pity than sympathy. A good lot of them were bullied. But does it justify the actions? No. So I guess at the end of the araw I don't find it morally incorrect to sympathize with a killer as long as that you don't proceed to try to justify or dismiss their actions.
Now fiction is a whole different story. I sympathize with fictional killers (like Regina Mills) all the time lol.
Criminal psychology always looks into the psyche of serial killer. Most killers are always mentally disturbed due to something, be it a mental birth defect or a horrible upbringing. Most serial killers come from poor families, abusive households, anti-social life and severe bullying. This later results in their mentality shifting into something that turns them aggressive, makes them socially awkward, and as a result, pushes them to take someone's life. It is a regular occurrence for people to ipakita sympathy for a killer, and isn't completely morally incorrect. While they don't agree with their actions, it is easy to see why the resorted to such actions most of the time. However, they still took a persons life and must pay for the crime. Only when someone admires the killer for their handiwork, which results in a disturbed romanticizing of the killer or a copycat killing is it morally incorrect