This will obviously differ from person to person, and in the cases of people with certain types of psychological conditions it might have no effect at all, or a reverse effect, but overwhelming guilt is often a big point, yeah.

A recurring point in terms of physical description seems to be that the rule of thumb is that if they did it with a weapon then a good way to portray it is to have them shaking to the point that this weapon, say, a gun for example, is almost uncontrollable in their grasp, so they either look like it could drop at any moment, or they could have to have it pried from their hands by someone. Showing the person has virtually been paralysed by the shock of what they've done can also help, and sometimes they're portrayed as spacing out to the point that people speaking to them have a hard time getting through. I think extreme shock is usually a good sign, and this is often shown by the person trembling uncontrollably. Another common portrayal is to have the person actually throw up after the killing happens, to emphasize how sick and nauseated it's making them.

Never killed anyone myself, so can't comment from realistic experience here. Mostly just giving what I frequently see as the way of portraying such an emotion, and using some of my understanding of human psychology. There's usually guilt, but how people handle it is different. Some people could become overwhelmed by that emotion and go into severe shock, while others might become traumatized in a way that makes them go completely dead silent. I don't think anyone will be able to give you a perfect way to write this unless they had personal experience, and even then it would just be a subjective situation; how they handled it and not immediately appropriate to everyone.

Internally, crippling guilt is a good start. This guilt can often lead to questions like "What right do I have to be alive?", "Why is my life more deserving to continue than theirs?", etc. Taking another life can also cause a person to have to come to terms with how fragile life actually is, which might change their perspective of life, death, or even the world depending on how severely it effects them. Some might come to realize their own mortality as a result, if they had never really approached the idea of dying themselves before.

I'm multitasking pretty badly right now, but if more comes to me, and chances are it definitely will once I'm able to think about it clearly, I'll either post again (*shock*double post*shock*) or I'll edit this.