The assumption here is that a murderer must have been driven to it by some sort of mental illness, that killing people is a sure sign that you're insane. But that's actually not true. When someone deliberately kills another human being--especially involving the sort of planning it takes to go on a shooting spree--it's a choice.

Yes, some murderers have a mental illness. But having a mental illness does not usually make you unable to tell the difference between right and wrong. Most people with mental illnesses--including very severe ones--are perfectly capable of understanding the implications of killing another human being. And once you get to the point that you no longer understand what "murder" means, much of the ability to plan a murder has vanished as well. I honestly can't think of more than a couple of cases where a person had a mental illness and committed a murder they would not have committed if they had been mentally healthy. Most of the time, it's not a mental illness causing murder--it's just that a murderer happens to have a mental illness.

I know most of us here have struggled with mental illness. Think of the times when it was worst for you--when you weren't thinking clearly. You might have been confused, sad, withdrawn, or even emotionless. But you didn't randomly turn homicidal. You were still yourself; you were impaired, distressed, maybe even angry; but you didn't go out and kill people, because your code of ethics includes "I don't kill people."

Yes, quite a lot of people think about killing a bully or a nasty boss or even a noisy neighbor. That seems to be quite unrelated to whether they actually do ever kill anyone. Unless it's an actual obsession, I'd consider it quite normal to think about killing someone who truly angers you. It's really very common to have "unacceptable" thoughts like that popping into your head. Don't sweat it. You might daydream about strangling somebody, but it's highly unlikely that this is an indication that you intend to do any actual strangling.