For my own suicidal urge states, I've been told that it's a type of psychosis. But everyone is different. Since my worst episodes happen when I'm mixed, it's sort of like the psychosis and energy of mania, plus the bleak "I must die now" aspects of depression. Because I think I "feel fine" too (and am mostly functional in some basic ways, like at work and in social contexts), it takes me a while to realize I might have some depression symptoms going on. It may feel like the images and thoughts about suicide are coming from nowhere, but for me, they really aren't. You have to do some digging though to find out why these suicidal thoughts or images are happening. For me, it's usually a bad stress implosion that I was in denial about for a long time. Often I'm in denial about my own suicidality, too. Even when I'm on the verge of attempting, or caving to the thoughts in an impulsive way, I can be insistent that I'm not actually suicidal. But if thoughts are driving you to kill yourself, then what else can it be?